I'd like to think Willow or Spike would make the leap (agree with JFAGhBro), with the possibility of bringing a few Wicca's/Warlocks like Amy and Micael Czajak into the mix. There's not nearly as much Occult movement as I'd like to see at the moment and well taking in account all the demons in the show, it wouldn't exactly be an unprecedented notion. Like Angel'sLuv states, most of the cast also have little to no experience outside the show, so they'd essentially be put in the wilderness once the show was finalised. Although a problem, I would hate to see the creators of Angel just quaintly fitting them in"¦ I mean even though I'd be concerned, sympathetic, yadda, yadda, yadda"¦ I'd still be very much against the formality. I'd want the best for show; Angel is Angel after all, not Buffy Mark 2"¦
I can't really see any characters making a move with the current Angel cast at the moment either. It's simply far too inflexible and implementing a "Buffy" member in my opinion would only make a hash of what I think Joss was trying to essentially make in the show being a "new product." Cordy and Wes are untouchable now that they've adapted with on-going scripts, but anyone else, well I think it's sadly too far, too late. They'd need to plan very early if not now if they'd want to keep any of the Buffy cast for Angel's 5th season. Otherwise it'd almost be impossible to swap any progressing characters (A) to re-model into another role (B). It'd be too much of a risk and could without a doubt ruin the original story arc the writers have surely been setting out for the next couple of seasons. Then again, back up plans"¦ Tara, the whole Oz epidemic. I'm not saying for sure, just throwing a curveball, planting a few seeds if you will.