I'd love to see a setting put together by cobbling together a bunch of existing Frazetta pics.
Angus MacBride's fantasy (LotR) actually wasn't nearly as impressive as his more recent historical work; I'd say get him again for a project, but tell him to make it historico-realistic in nature, and it'd probably be better.
Adrian Smith already does gaming, although more Warhammer stuff than RPG (yeah, Red Spire Press for getting in there!) but I'd love to see more of his work too.
And I'm glad Matt Wilson does the most excellent Privateer Press work, but I really want to see more of him across the board.