Three PvP Factions?

[Slamz]

The other way is to have diverging 'victory' conditions

This makes me think of DAOC and that central dungeon thing (which was implemented after I'd quit, but still).

Victory condition: get to the middle.
Once you have the middle, new victory condition: sack the dungeon.

Team A, having obtained the middle, no longer cares about beating Team C back to the newbie zone. They want the dungeon.

I do like the idea of simply changing the rules as you progress in such a way that it doesn't make sense to pound one team into oblivion.