Always an interesting question, and my would-be answer is constantly changing. I've got a fairly fleshed-out concept along with a couple others, and I've always told myself I'll go and make it one day. I even made a handful of sprites for one then forgot to back them up before wiping my drive. The thing is, I'm not sure I'd be able to enjoy an H-game I made myself for what it is - I'm already quick to tire of an H-game's content just from having played through and seen it all, I can't imagine what it'd be like creating, troubleshooting and tweaking each aspect.
Anyway, I think H-games generally work best when following the conventions of survival-horror games in mechanics, atmosphere and tone. Scarce resources and management, enemies that are challenging on their own and difficult when mixed, a sense of isolation and desparation. They'd be hard-ish sci-fi since that's my jam, fuck all this mystical magical girl shit.
Game type:
1 - Top-down ACT
2 - First/Third-person shooter/RPG
3 - Isometric turn-based tactics
4 - 3d vehicular-combat
Fetishes/H-Content:
Right there with you on bondage, tentacles, monster/beast, machines, soft-vore. Infested/infected/corrupted Humans would fit right in to these too.
Art:
Guilty Hell proved to me definitively that 3d animations can work great in an H-game if enough effort is put into them. If not that, it'd be sprites in a similar style and size to Parasite in City, where applicable - which could still work fine in an F/TPS given Doom and FPS of the era exist, though full-3d would be preferable.
Games 1 & 2:
Both are essentially the same and would only really have to differ in perspective.
Premise:
A ship believed lost on its mission to explore an uncharted part of space has suddenly returned. They haven't responded to hails and the ship is on a direct course to Earth. Your marine squadron is sent to assess the situation.
While attempting to board, tendrils from the alien mass nested to the ship shoot out and pluck your ship apart. All escape pods are grabbed and pulled into the mass, except yours - You crash into the cargo bay with nothing but a knife, pistol and a few magazines.
You learn through gameplay and deduction that the aliens are a hive-mind. Its infestation and corruption quickly transforms male humans into monstrosities.
Explore and fight through a spaceship overrun with hostile aliens. Your ultimate objective is sending a warning to earth, scuttle the ship, then escape.
Gameplay:
Essentially game 2 would be System Shock 2 or Dead Space "with Hentai". Game 1 would be the similar, but streamlined for a top-down perspective and a little more action-oriented.
In respects to H-games, they would borrow mechanics and concepts from Deathblight Guilty Raid/Last Demon Hunter, Angimanation, Angelic Apostle Liebe, Sinisistar, Unholy Jail and Furry Fury.
It would be relatively free-form, with various side-objectives and a lot of exploration potential. Different types of rooms/decks would afford different opportunities and threats. Each deck of the ship would have a number of alien sub-hives. They dictate the spawn-rate of enemies, what types of enemies can spawn, basically how fucked you are for being there.
Mechanics would include a "long" stamina system, ammo management, equipment maintenance, armor durability, buying and/or scavenging the resources to maintain them. Your ammo, stamina, weapon's conditions, other resources are all constantly ticking down to zero and your success will be determined by how well you do to avoid those zeros.
Combat would be comparable to tactical shooters of the bygone era, emphasising stealth, cautious positioning, using the right weapons, and planning your engagements.
Your stamina decreases passively and can only be replenished via "things" eg. beds, food and such. The lower your stamina, the lower your practical accuracy and the shorter the time you're able to sprint. There was one H-game that did something similar, but I can't remember its name. At zero stamina, you can still shoot, but can only crawl. Stamina-replenishing consumables take a while to begin taking effect.
Hitpoints would be a short-term thing, your stamina is consumed to heal your HP. Take X damage over Y time and your stamina is nullified, coming back after a while if you just managed to kill that last enemy from an incapacitated state. Most enemies have a HP-attack and a grab, some will only grapple, few will only attack.
Weapons would be the standard FPS archetypes, maybe with some variations - Melee, Pistol, Shotgun, Assault Rifle, Rocket Launcher, Grenade Launcher, Energy Gun, BFG. Most would have different ammo-types appropriate for engaging different targets, and can be up/side-graded. You'd be able to carry 2 + Pistol and Melee.
All firearms would degrade reducing their accuracy and increasing the probability of a jam, but can be repaired with maintenance tools you find or buy.
Melee weapons would be an instant-kill or stun (depending on enemy) when used as a counter-grab (think RE2make) but are only good for one or two such uses before they break.
H-mechanics:
Grapple systems galore. When grabbed, the game transitions into a turn-based mode, and you are defeated if HP reaches zero.
You have a limited number of actions and turns in which to attempt to escape the grapple. Which ones will work best depends on the particular enemy type and their health, your stamina, any equipment that might help you (eg. A knife). So, similar to Kunoichi Botan. Enemies that can join in on the grapple get closer each turn. Each action you can take would deplete significant amounts of stamina, potentially sacrifice HP, or use equipment. You can also choose to endure that turn to regain some HP but lose some stamina.
Each enemy would have multiple h-moves during grabs that each target different body-parts, each having multiple stages. The more you receive successful H-attacks to a given body-part, the more susceptible you become to subsequent ones (long-term, by a small degree). Defeat increases sensitivity across the board. The more sensitive the body part/powerful the attack, the more HP that attack depletes if successful. Armor/clothes present on those body parts would prevent them from being targeted during a grab, but can be broken by specific attacks. Some enemies may grab specifically to break armor.
Rape-on-lose would be a core part of the game's flow. When incapacitated (stamina 0) and grabbed, you're defeated. After the pertinent animation finishes, you wake up in a sub-hive or, if you're lucky, while being dragged to one, in a weakened state and without your weapons or armor. If you are unlucky and are at a hive, it's implied the PC has been impregnated.
If you have a knife, you can cut yourself loose. If not, you play another struggle mini-game. If you fail to escape before an enemy comes up to you, they play a unique animation and it's game-over.
Some enemies will "consume" the player-character essentially triggering this stage of the game. Weapons and armor are discarded/destroyed, and the enemy will take you to a different part of the ship depending on how long it takes to escape.
If you succeed, you must sneak and/or fight your way out of the hive room.
You have the option to return to where you were defeated to recover your gear, but weapons and armor will be somewhat degraded. Enemies may have dispersed but some may still be hanging around. Alternatively, there would be just enough weapons and ammo lying around that you could proceed from that point but would have to scavenge and use resources efficiently.
If you're defeated 3 times in a row without accessing the make-you-OK mcguffins (There'd be security stations dotted around that give you a reprive and maybe dispense some basic supplies), or reaching a given progression milestone, game over. Also game-over if you give birth 3 times.
There would be several drugs you can find or synthesize to affect different aspects of the H-mechanics - your chances of waking up post-defeat, your susceptibility to H-attacks, susceptibility to impregnation, etc.
Enemies:
Would be similar in appearance (and life-cycle) to the Flood from Halo and/or the Annelids from System Shock 2 and/or (some of) Dead Space's Necromorphs, taking various forms from small swarmer-types, to infested humans and aliens just barely recognizable as such, to large amalgamations of biomass, mouths and tendrils. Some would be capable of infesting mechanical equipment, some would lurk, ambush and evade, some would simply close the distance.
Bigger enemies could have armor and weak-points, while mechanical enemies are tough unless you have anti-armor weapons/ammo, but typically few in number and mostly static.
Game 3:
Think XCOM "With Hentai". This takes some inspiration from Angelic Apostle Liebe and Deathblight RPG.
I won't go too deep into it, but it'd be a sequel to game 1/2 (Spoilers!): The PC failed to scuttle the hive-ship, but managed to get a message out and divert its course. It crashlanded on a remote colony with a Planetary Defense Force in name only, with the nearest military ship X weeks out.
So, your objective is to defend against and eventually stamp out the rapidly expanding infestation within the colony. Once again, the aliens are setting up sub-hives, infesting structures and corrupting/capturing people.
You have a cast of say 15 (if 3d, probably randomized/customizable for anim compatibility) characters and can take up to 5 on a mission. Between missions, you use what manufacturing facilities are left to produce arms, ammunition and equipment for your troops, defenses for your outpost, research new equipment etc.
You have an strategy map showing the colony, your outpost and the alien hive/crashed ship. You start missions by detecting activity from the hive-forms - your standard mission would be interception of those forces, fighting them in relatively open areas.
If you fail to intercept such a group, they may abduct colonists and/or damage infrastructure, and/or infest an area, diminishing your production capabilities and inching the threat closer to your outpost.
These would be mission-types of themselves - If you reach a hive group before it completes its mission, you can prevent them from doing so - defend/rescue civilians, secure an important structure, clear out hive chrysalides before they can take form.
Stamina would make something of a come-back putting a soft turn-limit on your engagements - pushed past their abilities, your troops' performance will degrade and you will often be forced to retreat as a consequence of that, or simply by taking casualties. Troops require time between missions to recover their stamina. When a troop is incapacitated in a mission, you can use others to attempt to recover them to the dropship/APC/egress route - however this slows them down and leaves them vulnerable to engagement, so it may often be preferable to leave the casualty behind.
So while some would be provided through progression, the bulk of the Hentai would be from losing troops in missions - which would be something of an inevitability. If you abandon troops on a mission, they are captured by the hive. As time passes, you get (optional) H-scenes of what's happening to them in captivity. The longer you leave them, the more corrupted they become reducing some of their stats. You can rescue them, but such missions would be high-risk depending on where they were taken to.
You win the game if you survive with X% of the colony/population intact for Y weeks, and/or you destroy the hive ship.
Game 4:
This one is fairly straight-forward. Mechwarrior "with Hentai".
Premise is that more hive-ships have emerged and humanity is now engaged in full-scale war with the hive, who are employing infested alien vehicles and craft to win military engagements. Location-based damage, loadout/equipment customization, damage and armor types, all that good stuff.
This would pretty much be GoR, though RoL could work in this context - Your vehicle is disabled, the aliens capture you, you must then escape on foot to a new vehicle or friendly command-post, from where you complete the mission. I do like the idea of being "a cog in the machine", ie the battle goes on in your absence, but I feel something like this would end up too linear for it to be worthwhile.
