I was actually giving some thought recently as to how S6 would have panned out had Buffy stayed dead. I don't think it would ever have happened as a series, as mentioned above, I don't think that the networks would have risked it, but a few thoughts that occurred:
- Assuming that the other events of Bargaining remained the same (Scoobies trying to keep Buffy's death under wraps, Buffybot being discovered, biker demons invading town, etc.), Willow, Xander, etc. would have to call in some sort of reinforcements, and probably hide themselves somewhere. Perhaps a good opportunity for an Angel crossover, would be particularly interesting to see Cordy and the Scoobies meet up again properly.
- It's never really mentioned on the show whether another slayer was called after Buffy's death, or whether her line had already been passed on to Kendra, and wouldn't pass on again. I assume another Slayer would have been introduced in the event of Buffy's permanent death. Assuming that the Scoobies alert Giles to the demon invasion of Sunnydale, perhaps he uses the Council's resources to locate the new Slayer and bring her to Sunnydale as backup? There would then be the potential awkwardness of the new Slayer integrating herself into this group that have been fighting evil together for years, whilst coming to terms with her own new powers, and the rest of the Scoobies feelings regarding someone who they will see as their close friend's replacement - the group can be a bit insular, and Willow in particular can be quite unwelcoming towards people who she feels are edging in on "her territory", IMO.
- Regardless of the introduction of new characters, I think that Willow would have stepped up to fill the main character role, as someone the audience is familar with- Xander's role might also be increased. Willow's magic addiction would probably still be a major part of S6, though without Warren attempting to shoot Buffy, and killing Tara, whether she would go dark during the season is debatable.
- If the show could have convinced ASH to stay on as a full time cast member for at least one more season, I imagine they would have done so in this case, in order to keep as many familiar characters as possible in the show, and also to act as a bridge between the Scoobies, with whom he has been close to for years, and any potential new Slayer characters.
- I'm unsure what would have been done with Spike. On the one hand, there's not really a lot for him to do plot-wise without Buffy, as none of the other Scoobies, barring Dawn, seem to like him, and his S6 plotline basically consisted of a destructive relationship with Buffy in which he tries to isolate her from her friends, so in that sense, I think of all the remaining characters, he would be the obvious one to write out. On the other he seems very determined to fulfill Buffy's "last wish" when she was still dead, and help protect Dawn- perhaps he stays around for a while for that reason? And has a plot stemming from there.