I've got to say, I like the idea of having Zack in there with Cloud to help fight.
I think that final battle was meant to simply demonstrate the fact that Cloud defeated Sephiroth's control over him. However it can also be seen as Cloud is back in the lifestream fighting off the last bit of Sephiroth, and since Zack is still "alive" (The Maiden Who Travels the Planet), it would be badass to have him fight alongside Cloud, you could alter the final battle model cloud be Zack and have the Cloud you use throughout the game to be your controllable Cloud.
Sorry, I'm getting carried away