You're not the first to come up with this theory (I don't mean that in a critical manner); I think I first saw it over a year ago. It included quite the diagram explaining the relationships between Master Xehanort, Terra, and Xehanort as we currently know him. It posited that the being known as Xehanort (which, as many have recognized, is an anagram of "no heart" with an x thrown in) has somehow inhabited Terra's body.

There are quite a few holes with this, of course. For example, Terra appears in Kingdom Hearts: Final Mix + as the optional Enigmatic Soldier boss. It would be odd for him to make such an appearance if the Xehanort who showed up in Radiant Gardens was just a possessed Terra.

The thing I found most interesting about the theory was not actually directly related to the connections between Terra and Master Xehanort. It had more to do with Sora's relation to Ven, specifically in the manifestation of Roxas. It's already clear that the two are related by the keyblade. Although it is ultimately Riku who wields the Way to the Dawn (which Ven picks up in Sunset Horizons), it seems clear that it was originally intended for Sora (this is evidenced by the first game's emphasis that the Kingdom Key was intended originally for Riku and then the Enigmatic Soldier's comment that he did not intend for Sora to wield the Kingdom Key).

The Birth by Sleep trailers are already presenting Ven as something of an oddity. He even goes so far as asking Terra to "erase" him. The theory in question suggested that somehow Ven's heart ended up in Sora's body much in the same way that Kairi's did. This would explain why Roxas seems to resemble Ven more so than he does Sora. It also explains how Roxas, a being supposedly lacking a heart, can weild a keyblade, a weapon requiring a heart. Given what we currently know (or can at least assume), I don't think it's too much of a stretch to think that when Sora released his heart at the end of Kingdom Hearts to free Kairi that Ven's heart was also released and created a Nobody with a heart--i.e. Roxas. This seems more likely since a few of the extra scenes in KH:FM+ suggest that Roxas does, in fact, possess a heart.

I think that something like this may explain the connections between Terra and Xehanort. The one thing that Sora and Xehanort have in common is that they are the only two to produce heartless which retained their independent will. There is no known explanation for this, but it would make sense that this was the result of having multiple hearts within the same body. There are obvious holes in this theory, but it might explain the connection between Xehanort and Terra. Perhaps, somehow, at the end of Birth by Sleep, Master Xehanort's heart will somehow migrate to Terra's body to create the Xehanort which appears at Radiant Garden. Then when he begins experimenting and casts off his physical shell, you might somehow end up with Xemnas, Xehanort's Heartless, and the Enigmatic Soldier in the same way that when Sora released his heart we ended up with Roxas (a Nobody with a heart) and Sora.

More likely than not, the "Unbirths" will explain this better than any incomplete theories we can come up with given what we currently know. I do think there is a relationship between Xehanort and Terra, but at this point I believe we know too little to make anything more than base assumptions.