I agree, Xemnas could have an obession Aqua after all he does go to the room of sleep and as Xigbar noted to Zexion 'Talks to his Friend' (granted with current plot who Xemnas is talking to is up for debate.) and Terra's Wayfinder has yet to make an a re-apperance to make that connection.
It does give way to why Ansem is so Fixated on Riku to being with, he is in a sense Terra's successor (as Sora was almost Aqua's but through a loophole is Ven's) and The Xehanort in Ansem basically was trying to repeat Terra's fate on Riku.
Well Terra could have some unexpected advantage over Xehanort, I know i've said this before in DDD I was expecting that to happen when MX was talking about TAV, and i was hoping it would have triggered something in Xemnas...but alas that did not happen.
I agree with you, I'd love to see Terra/Aqua moments in the next game...but we know how that song and dance goes. but as i've mentioned before iwould still love to see Aqua vs Xemnas!
Perhaps BBS Vol 2 will be on 2.5 HD but as series of cutscenes (like Days) and we might get something we've been waiting to see!