Yeah, I agree. If they use the same characters as the main characters in too many series, it'll be over-kill... I don't know who else they could use, though... I doubt Chii and Hideki are going to star in another manga as the main characters (I don't know why, it'd just seem weird to me...) and I can't really see CLAMP using any of its characters from unfinished series like X or Legal Drug as main characters considering those characters don't even have their own stories complete.
I also can't see them using Subaru and Seishirou again (though I wish they would... *sniff*) because something about their story is just too.. You know... Their story is just so dark and depressing when you get down to it that I doubt CLAMP could make another series with them as the main characters without it still being dark and depressing, which really wouldn't make it that much different than it already is.
But I could be wrong! When I first read and watched CCS waaay back when I could never see Sakura and Syaoran as anything but card captors. But CLAMP portrayed them so well in TRC that I have no problem what so ever with Sakura-hime, Syaoran being an archeaologist (sp?!?!?), Touya being a king, Yukito being a high priest, Tomoyo being a princess, I like everyone's new roles! I also like the new relationships! I NEVER would have thought Sei-chan would meet Syaoran or Nokoru and the boys would meet the TRC gang. It's all really cool!
So, CLAMP could probably do a whole new cross-over featuring S + S or any other couple they like and it'd probably still be a hit and we'd all still love it! XD