a hypothetical reality in which angel returns to sunnydale permanently for s5-onward presents some very very juicy spuffel potential in s6.
buffy struggling with her lingering (but extremely complicated) feelings for angel at the same time as her blossoming attraction to spike. both vampires fighting over her while also being frustratingly attracted to each other (although neither will admit that such an attraction exists). tabula rasa ends with buffy making out with both of them in a shadowed corner of the bronze. all three fuck the house down together. etc
there's the guilt of sleeping with a soulless vampire COMBINED with the guilt of sleeping with angel again and risking him turning evil COMBINED with shame over how hot it is that she can't just settle for ONE ethically-questionable-to-have-sex-with guy.
and angel beating himself up that he's letting himself fall for buffy again, but in a much darker way, and in the process 'allowing' her to ruin herself with spike. and spike loving buffy's attention but being pissed off that he has to share it with angel (even though he regrettably wants angel's attention all the same and doesn't mind when hands wander a little too far when the three of them are getting intimate).
i just think that adding angel into the mix would have made things soooo much more fun worse