T.J.

Sure, Buffy was the main focus, but the show only worked so well collectively with the other prime characters that gave the show it's heart.

Absolutely Angel and/or Spike need to be part of any legacy characters who return in some way. They are too much part of Buffy's life to be ignored.

Angel/Spike weren't her love interests anymore when the show ended anyways, so there's plenty of other ways they could still be included without necessarily rehashing love triangles, and keep it focused on fighting whatever the next forces of evil are together.