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It's not a question of who would do MX but who MX would do.
And when he does...
When MX said he was planting his seeds, he meant it more literally than you think.
+both from destiny islands
+both obsessed w/ seeing other worlds
+both keyblade masters
+perhaps xehanort's heartless took control of riku because he wanted to be reunited w/ his son once again even though they were both engulfed in darkness
the only flaw w/ this theory is that master xehanort is like 60 but who knows maybe he's a very promiscuous old goat in fact scratch that he IS a very promiscuous old goat
try to refute it however unsuccessful you may be