Yes, I think "Service" should be your next priority.
We already have 2 poses for penetration. It's a lot of work, but the game despertatly need some service (bj/titfuck, footjob, etc...)
Then, and that's my opinion, and it might be a struggle as a dev.... Rework the face, for each characters.
Not in a "style" design, yours is already very good.
But add some emotionnal difference, like make Peach appear playful, or Tifa shy and hesitant, something to differentiate them.
Then add frowned eyebrows/ahego as they're approach climax, or after a couple of climax.
One easy thing to add would be bubbles with heart or moans or SFX but in a "comic" way, that would add weight to scenes, and shouldn't be too hard to implement.
Imo, those areas should be your first priority before any more skins are released.
TLDR:
-Service
-Differente emotion face
-climax/horny state
-some "comics/manga" writing here and then during play
Good luck.
