As pointed out in the LA Times article, it's not likely that the Marvel characters will come to any of the WDW parks any time soon because of the exclusivity agreement with Universal. Universal really doesn't have an incentive to get out of the contract especially when you consider that they just put a lot of money into upgrading Spiderman.
As for using other characters, the contract with Universal not only gives them the rights to the characters currently represented in the park, but also any characters in the same "family", the contract cites The Avengers as an example of a family, which gives them access to most if not all of the well known Marvel characters:
Storm Force Accelatron gives them all the X-Men.
Hulk/Captain America Diner gives them all the Avengers
Spirdeman obviously gives them that character and all the associated villains.
Dr Doom's Free Fall gives them the Fantastic 4
Of course there are other lesser know characters they could use, especially if they make movies about them.