Onyxia is awesome, and the features it provides by being on 3.3.5 offers something others do not. So, what if the progressions through Vanilla TBC and LK plays out on multiple servers instead.
It would look something like this:
Onyxia eventually releases all content, and players clear it. Instead of starting TBC, another realm would start, a 3.3.5 TBC realm. Players would be given one free "Character Copy" to get a clone of their 60 on Onyxia placed on the new TBC realm.
And then this would happen again when the LK phase starts.
At the end of the day, Warmane has a Classic, TBC, and LK experience all on the same 3.3.5 client. And players get to continuously enjoy the spoils of their work for each respective expansion.
Might be a lot of work, but could be worth it. Let me know what you think!!