I didn't like FFX-2(as much as X)but if an X-3 was to happen i would buy it without any second thoughts!
I also liked Rikku but a story about her doesn't sound very interesting to me.
If it was to happen i would like to see a more "dark" approach to the storyline.Something like,everything turns bad in Spira and a new band of heroes must form.Previous characters may also play some roles although mostly "behind the scenes".A mysterious ghost-character with a watersword also helps the group secretly...
Ok,i'll stop it here.Enough dreaming.

But yeah i would buy X-3 even if it was Rikkus story.