This would be the that body - it would be the final arbiter of what gets approved. The Tribune of the Plebs would not be able to approve anything at all, but would be able to unilaterally refuse to allow changes to be made.

The UN Security Council is somewhat similar in that respect, though comprised of more members. New matters are approved by majority vote, but the five permanent members can veto anything that gets approved.

My proposal is somewhat tongue in cheek, hence the names from the Roman Republic - but I do think the idea is sound.

An alternate, simpler implementation would be to simply require that all changes be unanimous. I like the idea of a Tribune of the Plebs though because it gives a second means of a voice should a sizable minority of members of the sub-communities strongly disagree with the direction of their representatives.