Not sure about you guys, but I would be happy to spend coins on extra spec slots. Let's say 10 coins per 1 extra spec for all of your characters? That way you could have many specs available including pvp and pve specs for each tree.
Since it is a quality of life upgrade, it wouldn't give anyone an advantage and would also give the server a large income stream. Many people would donate anywhere between 10 and 60 coins for this service.