What will happen to Isa in this game?

He'll die

You could just ask in the other Isa topic.

Hopefully he can be freed from Xehanorts grasp and reunite with Lea as friends .

He is symbolic of a dog, which means he's incredibly loyal, in this case, it might just be loyalty to his best friend, or to his master, xehanort, in my opinion, in loyalty to lea, Isa will sacrifice himself to save lea from death, sad but to my defense I claim nomura is one crazy puppet master with our emotions at the end of the strings

HE GUNN DAI,

but seriously,it's probably gonna be something sad since nobody can be happy in KH

He's gonna betray Xehanort and run off into the sunset with Lea. Calling it now.

I hope Isa tries to confront Lea and Lea shuns him, causing him to go into a deep depression

He goes to the good side because he thinks the old friendship betweeen him and lea is to important. 

Hope he gets trapped in the RoD for the rest of his life. He's pretty dumb.

He will lose to dark rescuer(Lea).

Death is waiting for him...