It's more, from my perspective, that the roster is not realistically going to be outside the 7-10 range, and when you take into account the mainstay characters who are clearly going to be there (whoever is Cap, Iron Man, Thor, probably Hawkeye), space fills up fast. You need a few women (the most prominent twofer available would be Monica Rambeau, but she doesn't appear to be on Bendis' radar; though actually, thinking on it, the current Black Panther is Shuri), etc. So, yeah, it usually does come down to picking one. I would have no problems with Luke and T'Challa both on the team (they'd bring appreciably different perspectives, one being an aristocrat the other a lower class workingman).