Correct me if I'm wrong,

To simplify the theory, she basically came here to re-write the history of 'Ultimecia' because of what happened to itself in the first place? That was her main motive?
And her realisation of becoming the master of the universe is what greatly motivated her to re-write her history. I mean, it's pointless to re-write your own history if there's no reward.

This theory is definitely better than the generalized way: players thinking of Ultimecia as some boring villain who simply wanted to rule everything by Time Compression and that's it. And this theory definitely makes the plot more interesting. But I still feel that the R=U theory holds the #1 spot.