Probably won't happen: WWE Legends under contract
The WWE Legends contract is, by design, a very flexible arrangement between the former talent and the promotion. The idea is that the Legends will be available for autograph signings, meet and greets, and the occasional (non wrestling) live appearance.
The Legends are typically free to work dates for other promotions, provided they are not in direct competition with the WWE (Impact wrestling and ROH are definitely on the list of prohibited promotions.)
However, All Elite Wrestling will likely be considered direct competition, and that means that the WWE Legends will have to choose between their cushy Legends gig with WWE or having a one-off appearance for AEW.
Given that AEW is trying to build a promotion based upon young, hungry in ring talent, and that the Legends are mostly incapable of actually wrestling in their current age, it seems unlikely you'll be seeing them in AEW.