Plot devices tend to make comebacks. Notice how Both CoM and Days don't really have Kairi but she played a role in CoM's plot if only because Namine brought her in, and Namine's conduct with Sora's Memories gave us a Kairi-lookalike in Xion's image.
And yet Kairi herself never made an appearance in either game save for CoM's credit roll.
Xion's being a plot device is exactly why the chances of her being mentioned again are high since she doesn't need to really be brought back, unlike, say, the Organization who'd need their Somebodies to be around (Isa and Lea, anyone?). Especially seeing how they emphasized how she shares Namine's motif of "Forgotten but not Lost" - and we all know how well even Sora kept "forgetting" about Namine in the FM+.
To me, that's how you can really measure a character's impact on the plot - the influence even without them being there. Riku had it in spades, Kairi had it even if sometimes forced to base SoKai through Namine and Xion (ew), and Sora FINALLY had it in Days. Namine had it more than enough if only because she basically gave us Days, and then in KH2.
Depending on her relations to Sora and Riku, she wouldn't need to make a full comeback if one at all - just be further explained.