I think any sort of fediverse-wide appeal community, or process, would risk compromising the whole point of the fediverse, ie, decentralization. The fact that admins have the final say on their own instances is part of what keeps the largest instances from controlling smaller ones and keeps the fediverse free of centralized control.

I mean, can you imagine a coalition of the largest instances coming together and telling a small instance “the appeal community agreed this user was banned unfairly, unban them or we’ll all defederate you”? Because I can imagine that sequence of events, if an appeal community got any kind of formal backing from the big instances, and that would pretty much end decentralization.