Now, giving from the recent trailer. Ienzo is helping Sora and co in getting Roxas back from Ansem the wises data computer. I'm assuming this would apply to bringing Namine back too, with the assistance of Kairi.

 

If this is how both Roxas and Namine come back, Namine is then tasked to get Xion back, but exactly how?, that's IF she returns the same time as R/N.

Well I think that Xehanort could recreate Xion.

As for Roxas and Naminé Sora needs the Key to return hearts.

Xion might pop out as a side effect of Roxas being extracted. I'm not sure how they're going to handle this considering Xion was retgoned and nobody knows she exists.