one of my most longstanding interests when it comes to alternate history is a succsessful hundred days. the problem being of course that you couldn't make up a more untenable position for a returning dictator to be in.
so, I've recently hit upon a rather novel idea, What if a different enemy, one more dangerous than an over the hill bonaparte rears its ugly head for britain to deal with, and napoleon is able to reclaim a measure of his former office by siding with his former enemy?
the trouble then is figuring out who might pose a threat great enough that britain would give up its rivalry to put down, and how do you put them in place without changing things so drastically that there is no deposed french emperor trying to make his return.
You might have just hit upon the biggest catch-22 of the hundred days right here. Napoleon's best (arguably only) chance to hold onto his throne in 1815 is probably to have some sort of falling out within the Coalition that makes it harder to defeat him and/or for his presence to be accepted, if only grudgingly. The problem with this, however, is that his return had a galvinizing effect on his former rivals, who immediatly united in opposition to him.
I'm not really sure how you can make somebody more repulsive to the crowned heads of europe than napoleon, either. There wasn't much love lost for the emperor in Vienna or Berlin when he was overthrown. Any acceptance of Napoleon is probably going to be made for reasons of realpolitic.
No matter what, Napoleon almost certainly needs to fight in 1815. it only takes one opponent to make a war, after all, and somebody is going to challenge him. His best hope is probably that he can win out, at least initially. After a few victories, certain powers may be more inclined to take his return as a fait acompli, and perhaps might see him as a useful counterbalance to the possibility of somebody else establishing themselves atop europe (ex: Britain or Austria backing him in the face of russia).
Given how long the various powers had been fighting Napoleon and how bitter the conflict had been that is a very tall order.
One thing does come to mind. Instead of returning in 1815 he is delayed for another year or two. The tension over the future of Saxony and Poland causes conflict between the western powers [Britain, Austria and Royalist France] and the eastern ones [Prussia and Russia]. Although unlikely, have some way that the eastern powers start winning heavily. Then possibly Napoleon returns to rescue the situation, throwing the enemy forces out of France and a de-facto alliance between Napoleon and Britain to finally drive the enemy back. Think its a bit unlikely that the two eastern powers could prove that powerful but only thing I could think of.
Steve
A few problems with this scenario. For one thing, would Napoleon really wait this long to return? Could he? I can't remember if the allies were actively planning to remove him from Elba, but there certainly were rumors to that effect. For another, I don't know how feasible his return would be during wartime unless the war went exceptionally badly for the french (although that would be quite the scenario). I don't know if Russia/Prussia could be so successful, but it certainly is possible, especially if their only direct opponents are Austria and possibly Bourbon france.