Neat, what's your project? I'm assuming something inspired by ImpG? I hope yours goes well, I had to pause mine till next year when I can save up more funds. Arts expensive. PaganWarlord and Anarking had a lot of good points that I concur with so I won't repeat their suggestions for mechanics.
As for content, thick women like Olivea and the Beast Woman are a must. Also ability to undress women piece by piece in the order I want. I often played this as just an undressing simulator, I wish there were more ENF reactions though and pose changes related to girls being embarrassed when ordered to undress. I was trying to do that in mine. What frustrated me and motivated me to do little mini mods myself was the 4 layers restriction, often a lot of clothing especially accessories would be bundled together, so I'd just use Paint.Net to reorder it in the way I wanted. I like to get a woman bottomless, or just in accessories. Girls only wearing shoes is great too but that's completely absent in ImpG, so full body views of the women would be an improvement.
Also more guest characters from other games or anime/movies, maybe as rare encounters. I loved 2B being added in the mods. I would love to be able to undress Jill Valentine in her Resident Evil Remake or RE3 costumes, and I'd love to undress Padme Amidala in her white Attack of the Clones outfit, maybe even a genderswapped female Clone Trooper would be cool too. Also the Dead or Alive girls. I was going to put these in mine but I had to scrap the idea because I wouldn't be able to list on Steam if I did, so I don't know your plans but free fan games can get away with anything. If not, at least make your game easily moddable, the way I see it, a game like this is kind of an ENF undressing sim platform, and if it was easily moddable, so many people's waifu's could be added to this so they can be undressed in the way we want them to be.