I think she definitely would have fitted back in. I mean, minor aspects of the season would have to be changed- given that she and Angel still seemed to have feelings for each other, I doubt Nina would have appeared - or at least, not as a romantic interest for Angel. But I think Cordelia would have filled a similar role to the one she played in "You're Welcome" in some ways- she is Angel's friend, she has been at his side from the start in LA. He can talk to her in ways that he just can't talk to the others- plus she's the only one who knows about Connor. As others have said, I don't think she would ever be happy about working for W+H, but I think she'd do it.
It might be interesting to see her and Harmony interact more on a regular basis, too, if only as a contrast to their friendship at the start of BtVS.
TBH, Angel Series 5 desperately needed more regular female characters. With the exception of the odd episode, Harmony was never really more than an amusing side character, nice as it was that Mercedes McNab finally made it to the opening credits after being around since BtVS S1. Fred was killed, and with Illyria (awesome as she was) I seem to recall that she wasn't really a woman, rather a genderless god inhabiting a woman's body. Not to mention that her story got pretty tied in with Wesley's. Eve appeared increasingly little towards the end of the season, in favour of Lindsey and Hamilton (and I still don't know why they brought in Eve rather than keep Lilah for at least part of S5, but that's another rant). I mean, I enjoy the male characters, but it was a pretty uneven balance there. Cordelia's presence would definitely have benefitted the show in that way, in addition to the other aspects of her character.