I'll tentatively answer WotC but with the caveat of it being contingent on them having certain people write it, preferably Monte or any of the other original members of the PS design team, and not certain members of the current WotC designers stable. Without naming names, I don't trust certain folks to understand the gist of the material enough, or care to do the reading on it, to be able to make the product up to snuff when compared to the Planescape books. Those same folks would be fine on other products, I don't question their ability to write enjoyable material at all, but from things I've seen I really don't feel that they have enough of a sense of the setting or the style of writing to handle it properly.
For instance, The Planar Handbook was Planescape pastiche at best, pure and simple, and at worst it was akin to screwing a corpse in a few places therein. Harsh but perhaps accurate. It had names and places sprinkled about but it lost the feel, lost the tone, and lost the style that Planescape had. And it was the style and feel of it all that made it a joy to play in and a constant and continuing inspiration.
I'd love to see Planescape back in print, but I'd rather see it not done than see it done wrong and butchered. I'd rather have no expectations than to have high ones and see them dashed.