Unions could also be involved, because they didn't seem to have much issue in calling the VAs back for Trap Team just to say 'Excellent","Whoohoo!" and "Oh yeah!". But I really have no knowledge on that,just that they have a say on what companies or media the actors can work on.
I'm sure the VAs from "The Simpsons" didn't expect to be working on the show 25 years later. Regardless of the obstacles, the quality of the bottom line effort will tell me if they want this to be in it for the long haul.