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:
Here is the confirmed list of heroes: