I agree with you JD. That would be a great way to have the tribes done. For Britain, I think that there should be 3 tribes. I know what the 3 are, except that the site that has the three names, is currently down. I'll try to find another source.
Now this next idea is a bit off the the beaten path. It's that the Gods will play a role in this mod. Well, sort of. There's going to be 2 small wonders, which we'll name Gaul God, and Roman God. Your civ can decide which god you want to worship. Each wonder can be built by researching. Although you can build both, each wonder will only work under a certain kind of government. So, you must research, build, and then change to to the God you want to worship. Building it will provide some bonuses.
Another idea is to add a converter unit, that is, a unit that will enslave a unit, and has a hidden nationality. This is a way to diplomaticly gain strength, without actually coming to war. A great way to weaken an opponet.