I'm afraid I'm with Eowyn on this one - it's certainly possible that Spike could show up, but I don't think it's necessary. If he does show up, I imagine it will be towards the end of the run, and it'll be treated pretty much the same way that all the Buffy/Angel reunions have been treated - ambiguous enough not to completely crush shippers' hopes that someday, someday... But I doubt it will resolve anything.