With all due respect to Monte's work, he was really a latecomer to Planescape--while he wrote a bunch of adventures, he didn't do much of the core setting material; only the Astral and Inner Planes books, Planewalker's Handbook, and the MC3. (Unless I'm forgetting something.) Compare this to some of the people I view as the core of the team: Colin McComb (Planes of Law, Planes of Conflict, Faces of Evil, Hallowed Ground, Outlands Primer, the Torment CRPG), Wolfgang Baur (Planes of Chaos, Planes of Law, In the Cage), Michele Carter (edited half the line), and, of course, DiTerlizzi.
Which, again, isn't to say he can't do great Planescape material; but his contributions were mostly to adventures and peripheral setting elements.
Personally, I'm not sure I'd want to see a Planescape revival. It was a great line, but it was also pretty much complete--there wasn't much left to be explored, and I'm not sure I'd want more detail on most of the already described elements. Planescape works better as an "open" setting, with lots of room for DM expansion. New rules would be nice, but most of those are already available (Planar Handbook, etc.).