oohhh that sounds so empirebuildinglike 
Planeshift isn't a RTS or 4x though. 
Yet, at least

It'd be interesting if a realistic group control system could be used, since most RTS and 4x games aren't very rp-like, but controlling groups, fighting over them, etc. could be interesting. Any chance that we can get worshiped as gods by the tribes? That could be... fun. After all, having an army is what some people dream of, and getting a group of rp'ers together as an army is kind of... difficult.
Tribes would give a whole new layer to evil players: right now, they're quite limited. But imagine an evil person exploiting/stealing from/leading to conquest/using/laying waste to/etc. the tribes.
Will the tribes be part of the faction system? It could be interesting to see how that works out.
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Having the tribes build these middle area settlements would be great, offering a huge new depth to the game: it wouldn't be static (as far as one day to the next, not counting GM events or players). Leaving one group one day and coming back the next to see a thriving village, and then coming back two days later to see them at war with a neighboring group, and then coming back a third day with some guild attacking and killing the town they built... now that would definitely set Planeshift out from the other games as far as quality of experience goes (although it's already set out from other games by quality of experience in many ways, both good and bad).