itch.io

In my opinion you hit the sweet spot with character design, realistic but not too much. I have played games with characters that were too realistic, and subsequently less attractive... the only real improvements to the designs I would say is:

For the male characters; maybe add a few small faint squiggles of either blue or purple for veins along the shaft, and to smooth out the transition between shaft and sack, a little bit more connecting skin I think would make it look less cartoony but not in a bad way.

For the female characters; maybe slightly change the proportions of the body/bust/torso, they do come of has a little artificial and I would have no problem believing they are androids has they are... Actually that might be another plot idea...

Future characters, I would recommend a variety of skin tones, builds (e.i. slim, athlete, lithe, and femboy for males, and petite, curvy, and somewhere in between for female characters) differing heights, but mostly I just recommend have differing personalities, making characters feel alive or rather stand out from the others will do this better than anything else, characters without this ability to standout from each other will come off has interchangeable skins of the same characters rather than different characters. This is probably the hardest to do but if done right will make the most difference in player choices. Having that variety of characters both in design but more importantly in personality offer the player base a wide variety of kinks to choose from has personality kinks are also a thing to consider.

I don't necessarily believe you need characters to be antagonists for this, however maybe adding characters with clearly antagonistic personalities can add a contrast, has well has for personality kinks of the type and character having their own issues.

For an example an antagonistic character that seem to try to sleep with has many women has possible and main preferring women in a relationship to destroy having a possible backstory of a really bad breakup including being cheated on and being used making said character have a tainted view of women following that with the possibility for helping him get past it. It gives a clear antagonist but keeps him human enough to not be this die on the hill of being an evil ass cliché, while leaves said cliché open, and other paths available.

Maybe adding some kind of character customization down the line with a start you own story, like if you start single and if not who you're in a relationship with... Definitely something long term but worth considering has an option.