What if Lincoln had not been reelected?
Concerning the war itself, not much would have changed. Lincoln was determined to win the war, before he left office, with or without McClellan's assistance.
Sherman would have blazed a trail directly to Va. in a much shorter time, than he did in fact, if Lincoln and Grant demanded. By the time McClellan assumed office the war would to all intents and purposes, would have been over, and little mac, would sooner stab his politician frineds in the back rather than his soldiers.
Post-war is murky, because it would depend more upon the make-up of Congress than McClellan. If there were enough hard war men in Congress, there would be a terrific 'dust up' over slavery, the 13th Amendment and Reconstruction.
P.S. IMO, McClellan would have no problem with emancipation, except he might try to get some compensation for slave owners (not likely to be successful, even if war weary , northerners would be unlikely to support paying slaveowners for slaves they had lost during a war they, the slaveowners, fought to save) he would probably not have supported the 13th Or the 14th and 15th, passage depending upon their support in Congress as they did in real life. The concept of 'Reconstruction' would be an anathama to him, I think and fought it tooth and nail. But, again, it would depend on the make-up of Congress.
P.P.S. And depending on the Makeup of Congress, McClellan (with his rigid personality and hatred of All Politicians) could have been much more likely And easier to Impeach.