We all know what happened in OTL to George McClellan. He was not the most successful general...i would say mostly because his his age and so much was thrust upon him.
....that said, what other roles would suit him. And given that he did seem to be a prodigy of sorts. He could have been an engineer in the Army, or perhaps even a Chief of Staff. Or maybe even a corps commander(most curious about this one, of him being in a subordinate position in the army)? How would McClellan fit in such roles?