Glad to hear that WoTC may be approaching other setting designers!
In addition to the Forgotten Realms, The 5 setting i'd like to see would be;
Greyhawk
Dragonlance
Eberron
Dark Sun
Mystara
With expansion options including;
Nentir Vale (as default setting)
Planescape (as Manuals of the Plane supplement)
Spelljammer (as Manuals of the Plane supplement)
Ravenloft (as Shadowfell supplement)
Maztica (as Forgotten Realms supplement)
Kara-Tur (as Forgotten Realms supplement)
Al-Quadim (as Forgotten Realms supplement)