Well start brainstorming more in case he asks for ideas for female enemies again. Personally I think the nuns are great because they mesh with the setting and their bondage technique is extremely unique.
That's not too difficult.
Looking at popular demands I've seen, there have been a few that have been followed, and a few that get completely ignored.
The first case being enemies that "swarm" the player, given how many approach them. So having an animation for multiple women approaching her would be nice. A second one would be "swarming" with different mobs, IE girl holds MC, forcing her to give another mob a blowjob or the likes.
Third one is melting pleasure. Having the MC give in to the mob to the point the MC is pleasuring them alone, like oral, etc.
As far as things not done, there's plenty out there. Size-play yuri, warp abilities like Amazon Brawl, etc.
There's good quality everywhere, just so few devs willing to invest in it due to personal goals. Not that it's a bad thing, it just means that you get "stereotypical" ecchi rather than "selective" ecchi.