I thought that would have been very intriguing especially if JW had kept Willow's redemption arc moving and had Angel mentor her. Maybe more of the soul curse that she had placed on Angel, twice, might have been elaborated on. It seemed like some kind of entity overtook Willow during the spell yet nothing was really made of that. I like Spike and thought he fit well on Angel but I think that Willow would have added a different dynamic to the halls of Wolfram and Hart. A magic fight between Willow and Vail would have been awesome in any case.
I would have added both of them just to fill the large Cordy-shaped hole in the cast, if I were Joss Whedon. I don't see Xander leaving Buffy's side or Giles's joining Angel so Willow and Spike are the few in the core cast that I could see working on Angel. I could see Oz appearing again however.
Also would have kept the place from being the sausage fest it turned out to be in the end. Hell, even Amy Acker was turned into a God King.