Let's talk design:

If affiliation is limited to one tribe, then I see two basic options: a) handful of large tribes (~50-100) or b) many many small tribes (~10-20).

Small Tribes:

There would probably be some minimum # of players needed for even the small tribes in order to have enough to regularly start games.  Each Tribe Game would be opened up to the tribe and joining would be first come-first serve since the tribe is so small.  Some cool benefits of the small tribe concept is the close circle community, more chances at playing a tribe match, and also a lot more options for playing large # of tribe matches (i.e. 1v1v1v1v1v1 tribe matches).

Big Tribes:

Basic idea here would be similar in terms of joining a Tribe Match but would probably require use of the picklist in order to keep things somewhat manageable and for players to have a decent shot at getting into the game.  The picklist could be a good combination here because it could potentially encompass the small tribe concept by partitioning the large tribe on a per-game basis.  Benefits of big tribes would be the larger circle community, more rivalry (think Duke vs NC), and more diversity in team dynamics per game.

My $0.02.