There has been no news of Shay joining the cast of the new 90210, it is unlikely that she will be a part of it but we'll just have to wait and see I suppose, though I don't think she will.
If they ever made a Charmed movie (which I don't think they will
) I think that Shannen would consider being a part of it, if she was asked. But that is my opinion and I can't read Shannen's mind. I don't think she would outright say no, and I would love love love to see Prue back in a Charmed movie.