Whether its nanao 'endgame' or mai 'endgame' (personally, I wouldn't consider myself a nanao fan, so my preference would be clear but that's besides the point) or harem 'endgame' going back to the original premise can only be good for the development of the story. Right now its kind of just monster of the week but instead of fighting monsters its having sex with lesbian prostitutes which is fine on its own but would eventually run dry.
Don't know why so many people seemingly want to ignore the mai plotline entirely to always be focused on Nanao? I don't think there ever could be a Nanao 'endgame' without it coming back, and just focusing on a girl that asumi doesn't currently have feelings for (and is basically using because she likes sex and the location of her house is convenient, and is being used in turn) because she's searching for someone else would just keep the same beats ad nauseam. Its clear there's going to be a development between her relationship with nanao over whatever happens with Mai when she does get to come back and it can only be good for the overall story of things.
Edited to say this isn't specifically because I like mai, I wouldn't know how their relationship is so I can't say I like them, just that I think its good to bring her back into the plot to develop things more. Personally I want senpai 'endgame.'