The repetitive dungeon IS pretty much the only thing after the story mode (which was incredibly short, there's pretty much only a scripted fight after that riddle).
There are a few scenarios/events that you can see when randomly entering the dungeon, but for the most part its a straightforward grind, with you unlocking characters/scenes/bonuses from points you get for delving deeper or fulfilling some criteria on each floor.
The game also boasts carrying data over (possible from the last 5 games, only the previous of which was a hentai game), so I guess a lot of the playability is trying to go through the -massive bullshit- that is the extra dungeon, while building up any potential bonus to be carried to the next game. Been stuck and kind of gave up on floor 46 for a while.
If you want the H-scenes, just select the unlock all H-scenes from the menu. Doing so changes your 'difficulty rating' to easy if you decide to publish your progress to the dev's site, but probably nothing to fret about.
