I would want it to be Thor but it will most likely be Spiderman which in its self would be awesome
Whilst Namora has said he's interested in including Marvel and Star Wars in KH3, I really don't want that to happen. It'd ruin the specific Disney feel of the series and clash jarringly with the JRPG Keyblade and Final Fantasy elements. I think they'll put some Pixar worlds in the next game though. That would be totally cool and in keeping with the tone of the story.