I've been reading some Napoleonic stuff lately, and I really love the atmosphere. I'd love to play a D&D style game, but with everyone as a member of a small unit. Just fancy uniforms, rifles, and bayonets.
Maybe they are scouting in Germany, and begin traveling through an old forest. Then they enounter an old hag, who just happens to use magic. Or the fae could appear. Or even something crazy like a naga could show up, as the leader of a group of Indian troops.
Sadly, I know of no OSR rules that do this. I know the armor system would have to be reworked, or everyone would be getting killed in the first battle.
Also, just which, if any, magical creatures would show up, and whose side would they be on?