Mickey seems to always make an appearance in every KH game and cg movie, even if i's a minor appearance.

 

Do you think he'll appear in Back Cover, similar to his appearance in X/UX?

There's always the possibility!  If Back Cover requires his presence, I'm sure he'll pop up somewhere!  Heck, he'd probably be a cameo!

There is always a chance, but I rather doubt it since it's about the Foretellers and what they did behind the scenes.

Calling it now! Oswald ends up saving Aqua if Mickey doesn't

There's a chance that they will include the possible interaction of Master Invi with Mickey in Back Cover. Other than that, I don't think Mickey would be related to KHχ unless there's a twist.

No I doubt he would, because that means he'd know about the keyblade war.

i expect him or his mouse head symbol to appear on a page of the Tomb of Prophecy. 

 

anything else would probably be a bit of a stretch. 

Well, the illusions of Mickey, Donald, Goofy, Chip and Dale did crash land in Daybreak Town, and every Kingdom Hearts title has featured a Disney character, I wouldn't be surprised if we get a Mickey cameo in the game. The real kicker would be if Oswald the Lucky Rabbit was introduced (seeing as he's Walt Disney's earliest creation: a reflection fo "the most distant past!).