If there's one thing that's slightly more agitating than Public Chat and the cost of the builders, the size limit of my tribe would fit just nicely in that category.
If there's a tribe who is successful and active, there should be a way for the tribe master or tribe to contribute to a Tribe Funds which could be used to expand the capacity of the tribe.
This feature could also be expanded further in which it can be used for tribes to give each other resources. Let's say my tribe hosts a contest for best-looking village. One person from each tribe would be a contestant and then the voting would be done on our website or these forums. After the winner is chosen, I'll add up the funds we put in for the contest and then gift it to the winner's Tribe Funds. /ramble
This is just a small brainstorm, and with just a small example of what else could possibly be done with it. The most important of the bunch would be the ability to increase tribe capacity.