@TitanAnteus and I were talking on discord about this. Most of the ideas were his.
I'll go over the conclusions we came to.
First things first, Zell's original goal on version 1 when FOBS was being done in FLASH was to make a glorified CG viewer where you ran around and found monsters to fuck. The premise was simple outside of the fact that you needed to do a small task to have them join you which was added later. Because of this he split up the monsters to be located in certain regions, and really only gave the player a single attack. Then he added powerups for more platforming, secret areas that were only reachable if you had certain powerups and metroidvania'd the game basically.
At the end of the day combat actually became the main way to interact with the game. You still solve puzzles and stuff but most of what you do is fight and if you lose you get raped.
The history of FOBS aside his current formula has a few flaws to it.
1. If you're not actively pursuing a certain monster girl's capture you will probably not lose to said monster girl and see their sex scene.
2. Some monsters are insanely weak so you won't lose to them unless it's on purpose.
3. You only meet monsters in specific regions.
Zell's monsters have basically all been "bosslike" in design recently because of this. You don't really get "normal" monsters anymore even though normal monsters can be interesting if done right.
Our solution is, first we need to add another resource for the player to worry about.
Something to make him weaker than he is currently. We also need to add a
modifier that changes how hard it is to escape a grab.
What if we added a
stamina bar. Running and jumping wouldn't take it of course, but attacking would. If you got caught while your stamina was low, you'd have a harder time escaping. You can also charge your attacks to take more stamina but do a bit more damage. The FOBS protag always attacks from his sheath so it makes sense with his iai jutsu. Imagine fighting the blond alice children which are supposed to be a gangrape fight (but if you play decently atm it won't be like that.) Instead of constantly attacking as you do now, you'll have to attack and worry about evading as you make sure your stamina never gets too low. If they catch you while your stamina is low it'll be practically impossible to escape. If you cum, you can't get back up again until your stamina's recovered so when the next alice is walking towards you, you'll be helpless on the ground as she goes her turn.
Another idea, is to have enemies cover each other's weaknesses. Jumping in FOBS is INSANELY strong. You can avoid most attacks by jumping, and while you're in the air reposition yourself accordingly. The Red Cat enemy in the desert is an anti-air enemy. If you put her in front of the mage cat, the mage might actually get a chance to launch her magic attack. Capturing enemies would be hard when you're dealing with combat in this sense so there'll be some areas that are just a solo monster to make it easier to capture. More things to reward the player with for exploring. Other good monster combinations could be the undine and slime girl(thinking of pushing her further back if she actually gets hit).
We also think there should be variable height jumping. Currently, in order to catch Kappa all you have to do is jump over her and hit her in the back. It's quite easy. What if the height of your jump was controlled by how long you held it? If you hold it for long enough you jump high. If it's a short tap, you just hop. While fighting kappa if you just do a short hop she'll catch you. If you jump too high she'll recover by the time you land. With variable height jumping we would not only make platforming more interesting but could give it combat importance. You can jump over tiny sticky spots so you don't get stuck and more.
Other ideas are level design related.
The mummy for instance is an AWESOME boss. The atmosphere and sex scene are amazing. I still remember how I felt when it first got released. Shit was hot. I think she was a bit underutilized though.
I think her area could make use of a long hallway. It would need a ceiling as she has now so you can't jump.
You find her. She wakes up... you attack her thinking you could beat her. It doesn't seem like you're doing much any damage at all so you run to escape.
You come across a pit of pink liquid with a plank of wood covering it so you can walk across. You have to try not to get caught by the purple swamp monster as you run forward, and the mummy is inching behind you. As you move forward you see other challenges. The pink mist is there ready to paralyze you with lust. Can't get caught by that. Maybe enemies like zombie appear to blind you, all while the mummy inches towards you.
If you get caught in sex by other monsters the mummy is still going to move towards you, and if she touches you, it doesn't matter if you're caught by another monster or not they're getting shoved out of the way as she wails on you.
We'd change some enemies to make them more interesting as well.
Zombie for instance would be a spawning enemy. They'll keep spawning as long as you don't break the glowing gravestone. Considering she's a gangrape enemy, it'll give her the chance to do that while also adding more platforming to the game.
Also the sitting girl is quite underutilized. It'd be awesome if she gave hints at how to capture certain monsters "for a price"
If you have sex with her she'll show you a picture hint of how to capture any monster of your choosing. This can't be changed until you find another one. Once you come on her/in her or wtv your max stamina/hp is halved and you always have at least 50% sperm, so you cum quicker. This cannot be healed until you save at home. We're thinking it might be too much of a punishment in order to get hints to capture monsters but it could be modified.
We're thinking of changing the monkey boss. Since stamina's added now we're thinking the way to beat her is to clash with her attack with your fully charged attack 1 or 2 times. Then jumping over her when she's attacking while tired and hitting her in the back. It'll be a really risky boss. In order to hit her, you need to expend a lot of stamina, so if she catches you... you're fucked.
Adding stamina would really change up the game and add a ton of design opportunities. It makes the character weaker without making the enemy stronger with no added resource like more animations or anything.
So yeah, those are our ideas in general to make FOBs more interesting as a game.