The scene just obviously points toward the end...
The three apprentices gather and take on MX in order to stop his plans...
MX and DS beat the three down, steals from Ven what ever it was MX was after, and summons KH...
Also, if MX had summoned KH before BBS, I'm sure he would have used its power to wipe out VAT...
It may give the impression that it's leading to their final battle, but the problem with what you're suggesting is we see Xehanort asking Terra to "Defeat the darkness" and "correct [his] mistake." There would be no sense in him fighting the three heroes when he wants their help. And it's that particular quote that leads me to believe the summoning of Kingdom Hearts is the mistake he speaks of, and the darkness to be defeated are the Unbirths. This also insists it's the reason for why Terra's peeved at Xehanort over Ven. In all likely hood though, we could both be wrong, and the battle actually takes place at the middle of the game, at the end of one of the characters' scenarios.
There really is no one else he can be referring to... Why call Ven "friend" in quotes, if it has already been established that they are friends...?
Simple. Ven lost his memory, therefore he's not the same person Terra knew him as.
Because it just screams "final battle"...?
Also, Ven's keyblade is destroyed in the secret ending...
You make a very good point here. And while I can't really argue against it, maybe, just maybe, Ven got his Keyblade fixed.
Not if they are against a nearly all powerful being...
Hardly all powerful. They would have done a much better job if they actually worked together and if Terra didn't have the martyr-like personality to charge head on by himself.
Taking Ven's heart...? (Assuming he to be a "pure of heart")
I really don't think Ven makes for a nice Princess of Heart. Although it would make some interesting fanart...