The ultimate spinoff would be with Wesley and Giles. Course there would be a lot of arguing over who takes turns researching and fighting.
After reading what could have happened in Angel season 6.

So much potential lost.
As for Willow, I never really supported her becoming gay in the first place, but the scenario of her meeting up with Oz again after Tara's death would have made for some interesting episodes. As it was, because of stupid fan outcry, financial matters, etc. we get stuck with Kennedy.

It would have been best for Willow to be asexual rather than be forced to be with Kennedy.