The giant creature seemed to be the source of magic in the universe... maybe the "goddess" that Willow invokes.
This explains why Willow was still alive 200 years in the future in ToYL, and why some (but not all) magic has returned to the world by then.
I can see Willow being the end of Severin. In most cases for him, draining someone of magic is like drinking a bottle of water. Even in Illyria's case, it would just be like drinking a big jug of water. With Willow, though, it would be like trying to drink a bottle connected to the ocean. He'll pop like a water balloon.
I also think Willow will be able to fix what's wrong with Dawn by re-charging the Key's batteries.
I'm afraid, though, that Willow is now too powerful to go on being a Scooby, basically part of Buffy's support team. I doubt she'll be a major presence in season 10. A separate Willow comic or graphic novel might be a very worthwhile idea, though.