After replaying Kingdom Hearts I on 1.5 and BBS on 2.5, I have an idea on where Terra might be.
First thing's first, Terra's "demise". His body was taken over by Master Xehanort, and some form of his will lingered after this possession, and as far as we know it's still hanging around the Keyblade graveyard even after Sora fought it. Birth by Sleep's original secret ending, Blank Points, showed Terra floating in some sort of black space with Master Xehanort. This seems to represent Terra's heart residing next to Xehanort's, implying it was still in his body but that he had no control over Xehanort's influence. Jumping from BBS to Dream Drop Distance, Xehanort(in Terra's body) has become a heartless and produced a Nobody, and both were destroyed. Once this occurred the original body was to reform, and in DDD young Xehanort states that he is waiting on his most future self, which winds up being Master Xehanort. I might be wrong on this, but I took this to mean that the reformed entity from the destruction of both heartless and nobody was Master Xehanort as an old man rather than in Terra's body. If this is the case, then Terra's true whereabouts seem to be unknown.
But perhaps we have been given hints as to where Terra might actually be.
Looking back at Kingdom Hearts I, it seems Xehanort has some sort of possession fetish, because Ansem SOD inhabits Riku's body in order to regain the form he cast off(I guess, its not too clear). After Sora defeats Riku in his skirt-wearing form, his body seemingly disappears. Later in the same scene, Ansem SOD shows up, and while Riku exercises a little control over him, he is nowhere to be seen for the rest of the game. HOWEVER, in kingdom hearts final mix, an added scene shows Riku immediately in the Realm of Darkness, beginning to fade, but saved by Mickey. So what's my point here? After Riku becomes possessed, similar to Terra being possessed, he is transported to the realm of darkness. So, its possible that Terra went through the same process once the entity know as Xehanort was fully destroyed.
So that's my theory on where he is. Here's a little more to it.
Riku immediately started to fade away when he arrived in the Realm of Darkness, but was fortunately saved by Mickey. If my theory is true, someone had to be there to save Terra, and it had to be right at the time of DDD, when Master Xehanort reformed. And Aqua is really the only candidate for this job.
So yeah, TL;DR Terra went to the Realm of Darkness and Aqua found him there, so I think that when they try to save Aqua, they'll find Terra with her. I feel like maybe I'm making too many assumptions, but Terra's whereabouts are so up in the air that I'm sticking with this.
First thing's first, Terra's "demise". His body was taken over by Master Xehanort, and some form of his will lingered after this possession, and as far as we know it's still hanging around the Keyblade graveyard even after Sora fought it. Birth by Sleep's original secret ending, Blank Points, showed Terra floating in some sort of black space with Master Xehanort. This seems to represent Terra's heart residing next to Xehanort's, implying it was still in his body but that he had no control over Xehanort's influence. Jumping from BBS to Dream Drop Distance, Xehanort(in Terra's body) has become a heartless and produced a Nobody, and both were destroyed. Once this occurred the original body was to reform, and in DDD young Xehanort states that he is waiting on his most future self, which winds up being Master Xehanort. I might be wrong on this, but I took this to mean that the reformed entity from the destruction of both heartless and nobody was Master Xehanort as an old man rather than in Terra's body. If this is the case, then Terra's true whereabouts seem to be unknown.
But perhaps we have been given hints as to where Terra might actually be.
Looking back at Kingdom Hearts I, it seems Xehanort has some sort of possession fetish, because Ansem SOD inhabits Riku's body in order to regain the form he cast off(I guess, its not too clear). After Sora defeats Riku in his skirt-wearing form, his body seemingly disappears. Later in the same scene, Ansem SOD shows up, and while Riku exercises a little control over him, he is nowhere to be seen for the rest of the game. HOWEVER, in kingdom hearts final mix, an added scene shows Riku immediately in the Realm of Darkness, beginning to fade, but saved by Mickey. So what's my point here? After Riku becomes possessed, similar to Terra being possessed, he is transported to the realm of darkness. So, its possible that Terra went through the same process once the entity know as Xehanort was fully destroyed.
So that's my theory on where he is. Here's a little more to it.
Riku immediately started to fade away when he arrived in the Realm of Darkness, but was fortunately saved by Mickey. If my theory is true, someone had to be there to save Terra, and it had to be right at the time of DDD, when Master Xehanort reformed. And Aqua is really the only candidate for this job.
So yeah, TL;DR Terra went to the Realm of Darkness and Aqua found him there, so I think that when they try to save Aqua, they'll find Terra with her. I feel like maybe I'm making too many assumptions, but Terra's whereabouts are so up in the air that I'm sticking with this.