Xemnas and Xehanort's Heartless both wanted to be whole.
Nomura has said as much.
Xehanort's Heartless accomplished this by taking Riku's body, as we know.
Xehanort's Heartless wanted Darkness and, I assume, suprime knowledge. Which is why he wanted Kingdom Hearts; he believed the heart of all worlds, (as we know, there is Darkness in every heart, so imagine) had the greatest, and most pure Darkness, and according to Maleficent, had untold wisdom beyond it. But as SA said, he got light instead, and perished.
Xemnas wanted a heart to be whole, obviously, but he wanted Kingdom Hearts to do it for him.
Xemnas:
Kingdom Hearts, hear me out. You are no longer any more complete without me, than I without you. (My guess is that Kingdom Hearts talks to him) Yes, but hear this. I will give you as many hearts as it takes. We will defeat the fools who hinder us. (Mickey says something) Hearts quivering with hatered (Donald says something). Hearts scarred with envy (Sora says something). Hearts burning with rage.
That fool Ansem said the heart was beyond his understanding, but it's not beyond mine!
This quote seems significant. Maybe it isn't, but here it is anyway.
Um, I can't really think of anything else to say.