She alongside the MC's sister have long been the most speculated but so it will be ineresting after all this time to see once and for all who finishes the main cast. Honestly im verging the sister these days as this deep into the story and characterisation of the cast i think it just makes far more sense Runey adds in someone who won't need near as much time dedicated to building up and integrating with the others as we've seen with say Autumn a complete outsider coming in fresh like last time (The Twins and Marie had pre-existing connections to other main cast when added so were smoother and quicker integrations vs Autumn who took a long time to win over many fans and Android's more recent isolated storylines seem to have tempered her reception popularity wise).
Still no one knows but Runey and he has never baulked at a challenge of continually adding in new faces and expanding the cast but my money is sister (esp as he's been developing the side cast far more as of late, that's spinning a lot of plates at the same time with more to spin yet from Julliete to the other university girls to a certain princess who i all assume will feature more eventually)
I am certain that he has said that none of the girls currently in the game will get the room, since I think he has no intention of bumping up anyone from side/background girl to main girl, so it will be someone we haven't gotten to know already.
I could be wrong, but it seems like the sister idea might have gotten scrapped, along with the third floor. So, if it is going to be someone we have talked about, the Wheelie seems the most likely choice to me.