Stranger things in Buffy have happened than Willow having a daughter, and it would open the door for Alyson Hannigan to appear without signing on as a regular or even recurring actor.
Maybe she could appear on Buffy a la John Stamos in Fuller House.
There are ways for Willow to have a daughter if she decided she wanted one.
That said, maybe she could play Willow’s daughter or other relative.