The sad part is, I can see this happening since as evidenced with Amy and Rory, Moffat doesn't seem to know when to say good-bye. I'm fine with Clara coming back for Christmas, give her a chance to have one final adventure and all, but that really should be her last episode. Her story is done, rammed into the ground, and finished. There's nowhere left to go, aside from an epilogue explaining what she's up to now and possibly that she's pregnant with Danny's child or something. There certainly isn't any reason to bring her back even for half a season.
I'm not really sure how her story is "done." She and Doctor left each other with some pretty big lies, and Clara seems to be headed down a pretty dark path. I could easily see her playing a role in another series.
That's why I say bring her back for Christmas, resolve all the loose threads left dangling with the lies they told each other, reveal she's pregnant with Danny's child or something, then have them say one last good-bye. There, that's all we need.
Moffat has taken things from one extreme to another. While the abrupt "I'm done travelling with you, good-bye" approach when companions left in the classic era at times didn't do proper justice to the characters, this "good-bye, Doctor, oh what the hell, maybe one more adventure. Or two. Or three. Probably call it quits after ten more adventures. Might stay longer" isn't helping much with the characters, either.