Well theres alot of good theories here but mines like this :
Remeber mickey fighting with Mx ( well i think they were fighting ) i think thats a event thats going to happen at the end of the game maybe just a thought why i think thats going to happen well remember in bbs he only had the star seeker well in the trailer he was with the Keyblade of the Realm of Darkness so what i know king mickey got the keyblade before kh1 but after bbs so i think thats a battle that happenes in the middle or in the end ( sorry for the bad english im from brasil )
I believe that MX/Xehanort will find a way into the Dream World to get to Sora and Riku as part of his overall plan. Just like Zhill05 said
But i think there will be a small battle just like Terra had with MX And Vantis ! in my opinion ! well thanks to the ppl who read this haha