Oh, they can bring in other characters from those movies. But they didn’t for a reason.
Hercules would be a fine addition to the game, but that movie was one of Disney’s weaker titles. James Woods’ performance as Hades made that movie, and turned Hades into one of the most iconic and beloved villains in the Disney canon, while Hercules… does not have that kind of esteem among the heroes. And Maleficent? With her flash and style and impeccable fashion sense, she is one of the most iconic villains not just in the Disney canon, but in all of animation. There’s a reason while Disney’s doing live action remakes of Beauty and the Beast and Aladdin and Lion King, when they did the remake of Sleeping Beauty, they named it Maleficent. Before Maleficent dropped, a lot of folks didn’t even realize sleeping beauty had a name.
Some Disney movies lean more heavily on their heroes. Many lean more heavily on their villains. And Disney villains are very much a self-sustaining brand that holds up as well as if not better than a lot of the Disney heroes.
Which is not to say there won’t be other characters from those titles, nor that other characters from those titles are a bad idea. But protagonist first is definitely not the way to go for every property. And in a lot of Disney titles, you can skip the protagonist outright and miss nothing. If they ever do 101 Dalmatians, it’s probably going to be Cruella de Vil and nothing else, rather than Pongo, Perdita, or Roger and Anita Dearly.