sheesh, it was just a suggestion, i mean Terra's Armor was an optional boss fight in KH2 Final Mix, so i figured why not Ventus since he is asleep and the game takes place in a dream.
I see it also as suggestion, yet still think it as unlikely.
Terra's Armor is just a bunch of thoughts that animate his armor in the real world which is driven by his pure hatred for Xehanort.
Ventus has no great rage or hatred, only sadness and enourmous pain from his badly crippled heart, which is the main reason his heart needs to sleep.
On that background I simply fail to see what would cause him even to have some sort of Sentiment in the first place and why it would then attack the one that is sheltering and protecting his own heart.
It just doesn't add up...also with Ven's personality who would not just wildly attack someone out of own volition.
I could honestly see something like that occurring. I mean, like you said, he is asleep and the game does take place in the RoS.
They somehow need to incorporate Aqua as an optional battle someway, whether it be in III or 3D. I mean, she's the only one of the trio that you never actually fight in any form, whether possessed or not. Terranort fights her, but you can't fight her as Terranort and Venitas fights her, but again you can't fight her as Venitas. The only way that you ever do fight her is in the practice duel at the beginning.
Ventus is inside Sora, not in the realm of sleep, buddy.
As we got to know in the realm of sleep there are those worlds that weren't saved at the end of KH 1, nothing was already confirmed that there will be also someone from the tormented ones shown in Blank Points.
Naminé is in Kairi, Roxas, Xion and Ven inside Sora, Aqua in the RoD and Terra either still with Xehanort or maybe with Riku.
That's arguable though as Terranort is more Xehanort then Terra and Ventus-Vanitas is clearly dominated by Vanitas. In fact, you practically fight Vanitas only with Ven's hair, easily recognizeable by the way Ventus-Vanitas holds the keyblade: Forwards, which isn't Ven's style.
As for Aqua as a secret boss...hmmhmm, maybe at the very end of DDD when Sora and Riku maybe end up in the RoD.
That would however again, just like with Ven, bring up the question why would Aqua want to fight her own allies?