We all know that everyone wants to see Galactus in the game, but how would Playdom manage to pull it off? It can't be a regular boss fight (just imagine Punisher, Spider-Man, and your agent defeating him easily).
Also, how could he even fit in the game. In the April fools joke they had him as a foot, but it was just a joke.
What level of difficulty would he be? People are already complaining about how hard it is to defeat bosses like Dormammu and Dr. Doom being to hard. Imagine how herd it would be fighting the Devourer of Worlds. If they made him easy people will also complain about him being to weak. Either way people will get angry.
Lastly if they do make him a villain in the game and have him as a tremendously powerful villain (as he is in the comics), imagine fighting him over and over again to master the mission.
Or to make things simple they could make him into a cloud  (that picture just proves that even Hollywood cannot get him right).