My suggestions (trying to keep relatively simple to avoid changing aspects of the game):
QOL: A "home" button that returns you to your room. I know it's only a couple extra clicks using the existing menu, assuming you don't get click happy and end up at the wrong location (Yes it's a bad habit of mine).
QOL: A shopping cart for the computer buying menu. Perhaps I'm doing it wrong, but currently if I buy an outfit and click back, it takes me back to the top level menu. Then I have to reselect the character, then the outfit, buy, click back, reselect the character, buy....... Current method works, but if I could buy character outfits either as a bundle or if you add a "back" button that goes specifically to the character you're already on, that'd be nice.
Random ideas:
Let the elves work at the bar as extra waitresses to perhaps bring in extra income to the Hotel. I saw suggestions for changing the whole purpose of the bar, but if you're happy with how it exists now, giving us extra options to "tweak" it would be nice. Perhaps giving us the option to buy themes / decorations so we can adjust how it looks (though I realize this would require tweaks to Moon's scenes depending on what you add). This would give them a purpose outside story / sex scenes like many of the other girls have and would give us an extra money sink to spend extra funds on.
More sexy locations in the hotel. We're currently limited to where we can interact with the girls, so if they're already doing something in a location in many cases we're kinda just stuck until they move. If we can interact with them in their chosen location like we do currently in the lobby (sex / blowjob on the couch), that'd be cool (IE: if someone is in the laundry room or bathhouse, we can interact with them there beyond basic hello). Adding to this: the ability to possibly call girls to locations like we do the dungeon. So if we want a scene in the bathhouse or our room, we can call them to it and interact with them there. Otherwise, many spots in the hotel are limited based on their scripted events. I can't think of the last time I went to the laundry room / bathhouse without being prompted.
Expand on outside Hotel locations we already have access too. Currently the pizza parlor for example? Pretty useless. Perhaps we can visit and "harass" Kali / Lucia while they're at work. Same with the girls while they're at school. As is, we visit the location 1-2 times and really never have a reason to go back.
Changing casual clothes. While dressing them up in skimpy outfits is fun, perhaps if we haven't set a specific outfit for them, they can rotate their own casual clothes(and maybe their sexy outfits if their stats are high enough). Off the top of my head: the twins go shopping for at least one outfit. Perhaps add 1-2 clothing options to the non-working staff that they can rotate through while wandering the hotel.
Maria's hair: would love the option to tell her to always have it down or up. I can't specifically think of any other character who changes their hair while you're in their room, but I think she looks lovely when it's down. A toggle would be cool.
Those are off the top of my head in an effort not to undermine what you've already designed. Hopefully nothing to crazy.