In future games if you play as Terra would you be able to use Xehanort's Keyblade

hopefully

It would be cool but highly unlikely and it wouldn't make any sense. Terra will probably use his own keyblade.

If he could use Xehanort's Keyblade, that would imply that Xehanort is still controling him, which would mean he is not on our side, preventing us from using Terra in the first place. So no.

Why if Xehanort could use a keyblade didnt he use it in kh2?

Because of Terra.

if we see MX in BBS volume 2, he would still probably be controlling terra, so there we would probably see terra wielding MX's keyblade. in KHIII, though, at that point i'm assuming MX and terra have seperated bodies, so if terra still had MX's keyblade, that might leave MX without a keyblade, so probably not then.

Hum... I dont see why he would.

only if hes still possesed