The next upcoming expansion to Marvel Legendary is "The New Mutants". This refers to a group of X-men, who has previously appeared in Legendary as special bystanders in the X-Men expansion.
New Mutants members Magik and Cannonball have already appeared in the game as playable characters in Secret Wars, Vol. 1 expansion and the X-Men expansion respectively, so our guess is that we can rule those heroes out. This is probably going to be a 100-card set, given the material.
With that in mind, here are our speculations:
The next expansion in line for the Marvel: Legendary series is Heroes of Asgard
This expansion will come out on February 26th, 2020 and will mark the 20th expansion to the game, which is pretty crazy.
Another expansion for the successfull Marvel: Legendary series is coming out in August. This time, we will have heroes like Scarlet Witch, Quicksilver and War Machine joining our ranks to battle the mastermind The Hood, among others. The set will also feature the Mandarin, which looks like a really cool Mastermind with some fresh ideas. When it comes to villain groups, the set will feature cool villains like the Dark Avengers.
Rio Grande games are releaseing two expansions for Puerto Rico! This is clearly a new direction Rio Grande is taking, ressurecting old games instead of spamming out Dominion expansions (perhaps we have finally seen the last one).
The expansions are The new buildings and The Nobles, both which has been released before, in 2004 and 2009 respectively.