I know 'The First' kinda played on the whole 'using the image of the dead', but I would love if someone that was a loved one coming back and being absolute evil.
I know Amber Benson didn't want to reprise Tara for reasons, but wouldn't it be really cool to have a character like thta be evil.
I know we have Angelus, but to an extent, Tara/Anya/Joyce/Wes as the villain would be far worse with a greater impact on the viewers.
Angelus, lets face it, was always there, but an evil one that has shown us they are good at heart would be awesome.