Senator William Knowland of California seems an obvious contender. One of the most powerful Republicans in the nation, if not the most powerful Republican in the nation, if there is no Eisenhower presidency starting in the 1953, he was well-known, and he had presidential ambitions of his own.
I don't know enough about Eisenhower to be able to make any judgment as to whether he'd seek the nomination again if he failed in 1952.
General MacArthur is very much a dark horse. He did have presidential ambitions, and for a brief period of time, he was extraordinarily popular. However, Richard Russell pretty much managed to make him appear a trigger-happy fool overly eager to jump to unwarranted conclusions with his hearings, so that when MacArthur did try to start gathering support for 1952, it was a damp squib and never went anywhere.