Hm. Prestige levels- that is, each time you pass wave 15, the enemies get harder. Maybe some way to scale the equipment so you can get more stats, increased XP of course as well. Would give a lot more replayability.
-As a bonus that only appears in Prestige Levels, if you DON'T fight or fuck Kalysea or Lily on their last appearance (after each wave 10/15 and you don't visit or you just leave) then they add an extra layer of difficulty to the regular levels.
-Kalysea just gets so horny that the wind starts blowing her musk onto the battlefield, spawning white clouds at random that bestow the same negative effect as during her battle. Perhaps when you DO go and fight her next there are persistent clouds of it in her boss arena, making it that much harder.
-Lily sends her minions to retrieve you, created a chance of having 1-3 bunny minions spawn each wave.
-As a semi-separate suggestion, a status effect from the bunnies and Lily- if they cum in your ass, your 'full of cum' counter doesn't go down (I think it does normally anyway) until you're full and use the 'Dump Cum' command- but instead of cum it's an egg. Unlike dumping cum it can be interrupted so you have to start again, takes longer, and causes more arousal. And possibly an extra fetish for egglaying. Morgana's vines might also plant seeds in you this way, though you've have to consider how to keep the two effects from interfering with each-other.
-Some way to fuck back would be nice, other than the existing scenes. Maybe a long-charge super-move that lets you instantly take down a target. Slow lunge, high miss chance perhaps? Perhaps limited to high-lewd characters so you're not completely boned when fighting the bosses at high lewd level. Charges based on cum taken, maybe?