Someone said it earlier on a different thread... the most obvious choice for a new commem is Martin Luther King.
They have already approved a Lincoln Silver dollar for 2009. Aren't five coins for him next year enough? (And then the Lincoln golden dollar comes out in 2010) Until this year I thought so too. But it turns out the Armistice on Nov 11th 1918 was just a negotiated cease fire and the shooting could have restarted at any time. The formal peace treaty that actually ended the war was signed Nov 11th 1921. The Peace dollar was intended to mark the end of the War to end all wars and its authorization called for it to be issued as quickly as possible after the Treaty of Peace ending the war was signed. At the time of the signing the design for the dollar had not even been approved yet. The designs were approved, models made, pattern dies, revised desigs, final designs and the coins issued all within about 45 days.