- Eve probably died and wasn't a strong enough character to be a regular.
- Nina wasn't interesting enough either and Nina would imply Nina/Angel romance and drama, which is a big no for me.
- Gwen is kind of a jaded but naive girl who probably couldn't handle Angel's world. I don't like her very much.
- Willow doesn't fit in with Angel. Her appearance on Orpheus was great, but it was kind of obvious that she was too perky for the show and that gloomy season in particular. She screams Sunnydale to me. I can see her as part of the cast though. Her sunny disposition could lighten the mood and a powerful witch fits right in with the Angel cast. Ats deals with more powerful forces than Buffy after all (Jasmine, the PTB, W&H, the Circle of Black Thorn).
- The ghost of Wesley would be super depressing and ruin his death a lot. I love Wes but I think he needed to die. It was an important part of his arc - his life becoming meaningless and even absurd in season 5 with his equally meaningless death pretty much summarizing Wesley's life at that point.
- Faith is right at home in Angel. Season 5 suffered from lack of important relationships, so her friendship with Angel would be needed. I think she's also a good influence on Angel and would make his character more interesting. Season 5 Angel was an unsuccessful corruption story, which doesn't go with the show or the character, because Angel can't be corrupted. Faith is always a reminder of what Angel should be doing - helping people like her - and of who he really is - someone who saves souls. She would make the show more like itself again.
So, my ranking of who I'd want to join the cast would be: 1. Faith 2. Willow 3. Wesley 4. Eve 5. Nina 6. Gwen.
In terms of other possible additions to the show that weren't mentioned...of the Buffy cast, Giles could be an interesting addition. He's smart and dark. He has the edge it takes to be in the show. Furthermore, with Wes gone, his watcher's knowledge would be needed. His aversion to W&H and even Angel would make his integration in the group difficult and tense. However, I think Giles would actually be good friends with Angel if he had the opportunity, so I can imagine a great story there.