Dear Lord, this thread is zooming by recently, so much activity. Not that I'm complaining. Even Sel is cooking with the constant previews. We eating good.
Anyway, the question about which LiL character to sex up has an objectively correct answer and that is (Moonflare, don't click on this) Kirin. Idc if you like her or not, she is the biggest freak in the sheets out of everyone (and not in the sense of "closeted freak" like Sana, God god bless her, or "sociopath freak" like Nodoka, God god please don't bless her) and would be the most fun, if sex is all that you were looking for.
I would potentially allow Chika (since she is similar to Kirin in that regard), Touka because of the titty bonus, Sara, Haruka, Rika or Maki for experience or Clown Yumi for the novelty.