I like the idea of more roommate options, too. However, I don't think every non-married adult NPC should be an option.
It's set up very clearly that Linus likes his lifestyle (he even mentions being worried you were going to offer for him to move onto your farm at one point), so I don't think he should be an option. Willy, Gus, Clint, Harvey, and Marnie all have apartments/rooms in their shops that would be weird to leave. Abigail and Penny both live off their parents, so it doesn't make sense that they could move out on their own without the benefit of marriage. The Wizard has his own stuff going on and I can't see why he'd want to move in with the farmer. Haley or Shane could really go either way, I think.
Sam, Sebastian, Maru, and Emily would be perfect for this. They each have jobs (that they should keep after becoming a roommate since they aren't theoretically going to be taking care of the house and potential children) and most express a want or at least a consideration to move out. Elliot mentions that he's living off his savings and running low on funds before his book sells, so I suppose it could make sense for him to want to split the bills with someone. I don't remember how Leah pays the bills, but I would expect her to be struggling at least a little with how long it takes her to start selling her art, so that would make sense too. Sandy makes comments about missing the Valley and the farm is right by the bus she'd need to take to get to work. The Dwarf I imagine would be similar to Krobus.
I don't know how one asks Krobus to leave if they decide to get married, but I think there'd have to be some sort of eviction option should the farmer choose marriage after having a roommate to free up the spouse room and outdoor space. Should the roommate space be slightly larger to include a bed or doorway (that is remarked on as leading to a bedroom), or should the farmer be expected to provide a bedroom for the roommate?
