...how much Terra's hair looks like a shorter version of Xemnas' hair? I'm not sure if this theory has been brought up before, it probably has, but here's what I think. (New lurker to these forums and didn't see the theory when browsing.)
In the previous games, we find out that Ansem had found Xehanort (young) with little to no memory and took him on as an apprentice. Xehanort eventually became quite interested in the darkness, and using Ansem's name, lead the rest of the apprentices down that path that eventually lead to the creation of Xemnas and the rest of the founding Nobodies within the Organization.
That youth has had a known history of using other's names. It's possible that 1) he hadn't really forgotten anything and just wanted to use Master Xehanort's name, 2) Master Xehanort was the last/only thing he remembered at the time Ansem found him and took the name as it was one thing that was familiar to him.
Terra must be Xehanort/Ansem/Xemnas.
I'm still under the impression that heartless and nobodies are not a new creation. It was only found that ARTIFICIAL heartless had been created by Xehanort (young) and company. Could it be that Master Xehanort was the first strong Nobody, or at least the first prominent Nobody in current KH history? KH1/KH2 had no X-names unless they were nobodies, with the exception of that person, and if Terra took the Master's name, and the Master's history is still shrouded a bit in speculation, it's very possible that he was a Nobody.
DISCLOSURE: I've only played KH1 and KH2, and watched the cutscenes of KH:CoM and KH2FM+.
