Many years ago I wrote a "Vikings & Dinosaurs" setting for Pyramid magazine, and I think about re-visiting it every now and then. Thanks to Infinite Worlds' banestorm McGuffin, I was able to drop some medieval Norwegians into the South Pacific of a timeline with no K-T extinction, and since it was part of the IW setting you could even have modern-day time travellers drop in and join the party.
Murderous as the Taiping Rebellion was, it would make a great background for the Mythos!