I think if they're going to promote a GSP fight that's not going to be for the WW title, it's gotta be a major Superfight and the considering the possible options at MW, if they don't want to throw him in there with Anderson, there are very few fighters who offer legitimate big game opportunities.
An Akiyama showdown in Korea would be ridiculously huge and while Vitor, Ace, Hendo and Franklin are pretty massive for MW, and they're the biggest superstars in the division and could potentially provide excellent and entertaining matchups vs. GSP.
However, my pick would probably be Anderson, because a loss to him, coming up in weight would be the BIGGEST potential fight possible and doesn't terribly devalue GSP's stock and drawing power.
MY MMA RANKINGS
HW Velasquez Cigano Werdum Cormier Bigfoot Barnett Mir Browne
LHW Jones Machida Evans Hendo Davis Goose Teixeira Shogun Mousasi Bader
MW ASilva Rockhold Belfort Weidman Jacare Okami Kennedy Consta Belcher Boetsch
WW GSP Hendricks Condit MacDonald Diaz Maia Fitch Kampmann Ellenberger Pyle
LW Bendo Alvarez Maynard Melendez Thomson J.Miller Diaz Pettis Chandler Grant dos Anjos
FW Aldo Mendes Curran Lamas Frankie Zombie
BW Cruz Barao Fernandes Faber Moraes