Alright, I guess I really underestimated the file footprint of the tilesets, so I've done some more aggressive trimming, and managed to knock that zip down from 91MB to 50MB. Still gynormous, but less wasteful, I guess.
I also want to start making a list of ideas. I'll put the specifics in spoilers, in case people prefer to be surprised, but I will say up front that I'm not into most of the dark themes that people put in hentai games, like rape, torture, mind breaking, incest, etc. The characters are on a fun, sexy vacation, and we're on a fun sexy vacation with them. Also, making new character models & rigs is kind of a big deal, so I can't do a new character for every little sub-adventure, but it's also not that big of a deal, so there probably will be a few extra characters. There are NPCs that come with the Polygon Samurai Pack that the environment graphics come from, but I don't think they'll be useful for sex scenes.
Spoiler (click to show/hide):
Radishes: The girls are intrigued by Ninja-land's native daikon radishes, and buy some to shove into their various holes. I have various other props which could be inserted into their various holes in the same manner, but I don't know how far I could really take this concept.
World's Oldest Profession: The girls need money, or need local currency, so they sell their bodies for some extra cash.
Different Outfit Requirements: I'm thinking that NPCs might react to the girls differently depending on what outfits they are wearing. For example, a shopkeeper might freak out if they're running around naked and not sell anything, or wearing the ninja costumes might get them into the secret ninja club.
Public Exhibition: Streaking down mainstreet, maybe the girls dare each other, or this happens if you're walking around with no clothes.
Punishment/Curse: The girls get physically punished or magically cursed for accidentally disturbing something or maybe for constant sexual deviancy.
Secret Strip Show: There's a secret or exclusive strip club that the girls want to get into, but they need special knowledge, a special pass or lots of money to enter.
Big Boob Potion: Secret ninja magic potion makes their boobs grow huge.
Tentacles/Monsters: The girls find and fuck a tentacle creature (alien/octopus/vines/etc) and/or some kind of other fantasy monster, like an ogre or huge bug. I have a few such things ready to use.
Drugs: The girls acquire some recreational drugs (or just booze) that make them do something they normally wouldn't. Although, it sounds like they're naturally pretty uninhibited.
And so these various episodes might tie into one another. Like maybe the girls do prostitution to get money to buy radishes, or getting the ninja badge to enter the ninja club where they find secret ninja magic big boob potion and secret ninja magic marijuana.