Room One: Observatory (a remake of Celeste's Observatory in previous AC games)
Room Two: Museum Offices (basically a counterpart of Celeste's room for Blathers)
Room Three: Research Ward (basically a science research room and testing facilities)
Room Four: Nothing yet, just storage.
I can't decide what I want my Museum rooms to be. I was thinking about just moving the Observatory and possible the Research Ward to an alternate character's house, but I don't know. I have the opposite issues as you; I have way too many ideas for rooms I want to make in the Museum, haha.