Fictional Fans

It's also tough because usually the codependent people *are* the main cast, so bringing a new person in usually means an OC/crossover character/AU, and those are always tricky.

YMMV, but in the versions I mainly enjoy, it's usually either a new-ish canon love interest or someone else in the cast who just isn't part of the main group. The version where I first encountered it enough to realize that I liked it was in SGA when people would focus on Keller in Keller/Rodney stories, and specifically on Keller making friends with Rodney's team, or with John specifically. There are several other canons where I've run into it since (or realized I'd run into it before), often with villain/hero pairings where a big part of the story focus is on the new villain love interest having to get to know the team and deal with their distrust that s/he is going to break Hero Character's heart.

You saying this about OCs/crossovers makes me realize that I have those categorized completely differently, because for me the appeal of this kind of story is mainly about building up connections within the main group that you never actually got in canon (someone who's a relatively minor part of the cast, or in a different general section of the cast, getting accepted into the main group). It's a slightly different kind of outside POV than the outside POV of an OC or someone else who canonically doesn't know the team at all.