I have a few "What if's"
What if the entire ground at the battle was flat?
What if the Prussians had not have arrived?
What if Napoleon had decided to not take over Europe again, but to return to France to live in peace?
Those are just a few I sometimes wonder.

4409053
1. I doubt Wellington would've selected a battlefield that was flat given his experience in the Pennisular War, and he was well known for planning effective defenses. Had he done so it would've left his entire army vulnerable to artillery and Calvary, and likely would've resulted in his army sustaining heavy casualties and could've cost him the day.
2. That is the one moment where the tide could've turned. Had the French Forces that had been pursuing Blucher arrived before him or Blucher assumed that the British were fighting a skirmish and continued on his way, the French likely would've overwhelmed Wellington leaving him with no choice but to retreat or surrender, and the result would've likely been a repeat of Austerlitz in terms of propaganda since it would've been seen as Napoleon defeating the one man, Wellington, who was ever able to fight him as an equal.
3. I doubt the Allied powers would've allowed that. Part of the truce agreed to after the fall of Paris was that Napoleon was banished from France forever. When Napoleon escaped from Elbe he effectively broke the cease-fire and it was made all the worse when he toppled Louis XVIII and began rebuilding his Empire in France. Remember Napoleon was still the most feared man in all of Europe when the Seventh Coalition was formed, and the very thought that he'd regained power and his army was enough to convince the Allies to resume the war. The only way Napoleon would've been left in peace would've been had he remained on Elbe