Let's remember that Terra was *(Spoiler)sabatoged(end spoiler)* during his exam by Xehanort just like Sora was.
Also Terra was manipulated by Meleficent and the seducing nature of MX to whom at that time was believed to be a good guy so Terra wasn't evil only misguided. Even Eraqus thought at first MX was good and had gone to seek out the cause of the Unversed. Terra wasn't evil he just didn't know the whole truth and let darkness in by a lack of confidence in his actions.
Mm! And I kind of hope we get a secret boss like that. I mean, when I beat 358/2, I was at least hoping for something Birth by Sleep related, but all we got was an annoying, giant wyvern who of me and my group of friends couldn't quite beat together unless we had an incredible amount of patience!
But enough of that, I'm kind of glad to see Disney characters outside just the movies popping up as well! I really enjoyed Runaway Brain, showing a side of Mickey most people aren't used to seeing now a days! Julius is pretty boss.
I think that Zack needs to have a bigger role in the series. If they incorporated a remixed version of Zack, Cloud, Sephiroth, and Aerith's story it would really hit home with the fans of FF7. Also I think Vincent Valentine would be a good character for the Underworld if you know his story. And Jack Sparrow and Barbossa used guns so that wouldn't be a problem. How cool would it be to have Vincent changing forms in battle?