Planescape would be cool, but I've already got most of the old stuff already, and the Manual of the Planes, so I can pretty much run a 3.x Planescape campaign anytime I want. And, planar stuff was moved back into the core material, instead of separating it.
Al-Qadim would be an good possibility as well, writing it as a FR sourcebook could work, or it could be set up like the original, as a setting that was officially Realms, but could be dropped in any world. Besides, al-Qadim was a very popular setting anyway, iirc.