For the North: Not firing McClellan a lot sooner.
For what it's worth, I think the problem with the idea of firing McClellan sooner being in any way beneficial is a simple question of who and what the alternative courses of action were.
Basically:
1) The men who were considered to replace McClellan were not exactly men who set the world on fire themselves, and indeed both of the men who actually
did replace McClellan at different times (Pope and Burnside, depending on how defined) pretty quickly committed a major error - one resulting in a major
defeat - which would not have happened under McClellan.
2) Given McClellan's generalized methods through his time in command, and the challenges he faced, then to replace McClellan with someone else (at any point in the war) and produce a significantly improved outcome requires at least one of these to be true:
- The Overland campaign produces
quick, decisive results within a few months despite facing an enemy of comparable total strength.
- Richmond could have been easily taken by the Peninsula campaign with only the resources actually assigned to that campaign, but McClellan did not take an opportunity that actually existed.
- The Maryland Campaign offered a significant chance to
destroy Lee's army and Lee did not notice this risk.
- Attacking straight at the Confederate army without manoeuvring for advantage can defeat it decisively.
Now, obviously, if firing McClellan replaces him with a general who is able to perform better with the same level of resources relative to his opponent, this obviously means the outcome is better. But I don't think there's any general in the war on the Union side who has that ability in any proven way.
By that I don't mean that McClellan was the
best general, instead I mean that no other Union general does anything while commanding that makes them clearly superior. There are others who do well under less trying conditions, but none I can think of who face an equal challenge and come up better than McClellan did.