I don't know if Chii will have a huge roll to play in the future of Tsubasa, actually. Somehow I feel like she was only put into Tsubasa because she was a popular Clamp character and the series was just starting out. While I'd like to see Hideki, I don't think he and Chii will get a great deal of focus. I'm wary of the idea that the group will go to Celes eventually (though damn would I like to see it) and if that doesn't happen, all Chii is set to do is warn Fai of Ashura's awakening. I don't know if she'll be able to physically appear or what the method of warning Fai will be.
Though if it does occur, all of the Clamp couples that have made a re-appearence have been slightly different than their original counterparts circumstance-wise, haven't they? Sorata and Arashi being together and happy, for example. How much can Chii and Hideki change? Make them more angsty?