1813 and 1814 were disastrous years for the napoleonic army while France could again recruit in France, in Belgium, in Holland and in Italy of the North which were French departments. Besides she(it) had Polish, German, Italian and Croatian and Swiss contingents. Nevertheless, she(it) was given up by the Prussia, Austria and Russia. In 1815 Napoleon arranged even no more reserves of Holland and Belgium. His(her) allies of Germany, Holland, Belgium and Italy had returned against their former(old) brothers of weapon (the majority of the soldiers of Wellington were not English) and Poland did not exist any more.
On the internal plan, Napoleon had lost his main supports in France and the conscription had become unbearable. In the spring, 1815, the beginning of civil war was reborn in the West of France. The best French troops had died in Spain and in Russia. The best marshal (Davout) is held in Paris and Berthier the best staff chief died. Many officers will not follow Napoleon and any are even going to betray him.
How Napoleon would have been able to succeed in 1815 with an army of 200 000 men against 1 000 000 what he had not managed to make in 1813 with 180 000 men against 330 000 ?
If Napoleon had not been beaten has Waterloo it would then have been beaten later simply by Russia or by Austria. The only thing that Napoleon would have been able to hope would have to stay at the head of a country reduced to its borders of 1789.
On the other hand, the political exploitation(operation) of Waterloo was immense for England because she allowed him(her) to become the first world power of the XIXth century. The sun never lay down(went to bed) on the British empire.
As for what other battles might be as or more important; Gettysburg! If Lee is able to destroy the Army Of The Potomac, as planned, there is the possibility Lincoln does accept the Confederacy's bid for peace & the country is divided. The USA is plunged into a European style 100yrs War as North & South battle during the race west. Native Americans are likely wiped out. USA's participation in both World Wars is nonexistent. That may mean nothing in WW1 as the German's were on their way out by then, but it likely meant possibly another year of war & 100s of thousands more casualties. BUT, without at least USA's aide, it is highly unlikely Britain stands against Nazi Germany. Russia stands alone against all the Axis.