I think it was never pitched as an idea because Xander, Willow and Buffy somewhat comes as a package deal, I don't see the writers separating that group. Same somewhat applies to Giles but they have more room with him because of Ripper.
Anyhow, I obviously love the idea of a Willow spin off (who am I kidding, I would even watch an Andrew spin off at this point). But I do think Willow would make a good standalone, I didn't read WW but from what I've heard it did very well with sales.
As for the show itself, I'd just like something focused on exploring the magic world, so something that deviates a bit from the whole demon thing, and instead could deal more with dimensions and the mythology of magic. So off course add a few wiccan or shaman characters. They could have her on a mission to restore the slayer line after it is broken at some point between the Buffy comics and the Fray universe, and it could deal with DW of the future. Maybe have some time jumps. Just random ideas, there's quite a bit they could do with her.
Not too sure for the themes, I'd like it to be based more on mythology and a different world than what we got with Buffy or Angel which dealt with vampires. Maybe introduce Tara as an entity (I don't want to say ghost) that guides Willow. But themes, I'm blanking. I think they could have some pretty unique themes though compared to Buffy and Angel.