What do you guys think of a tribal upkeep system? Such as one that requires resources and crafted items be offered to the gods of Xsyon each day? If tribes fail to make an offering the gods lose some faith in them. If it drops to zero they lose control of the territory. Effectively destroying the tribe. The larger a tribe becomes the more organized, and efficient they need to be in order to maintain control of their territory. This would be a great mechanism to remove poorly organized, or inactive tribes, and also reduce the size, and sprawl of larger tribes.
This system would also mean wars are fought to weaken, but not outright destroy opposing tribes. It becomes more a battle of attrition rather than wanton destruction. So raids anytime during the day would be possible. As opposed to scheduled raids which I think are kinda lame.
Also this would prevent large zergs from destroying every tribe in their path. They could weaken them greatly, but they could easily survive any massive raid if they can make an offering on time.