The Teutonic Knights would be awesome. They were definately a distinctive faction that played a large role in Eastern Europe until Tannenberg.

They had awesome knights, and also all sorts of mercenary infantry from Europe.

Although they were almost all German by birth, the order quickly established that it owed no allegiance to anyone but the pope. It acted on its own, like any other kingdom.

It could be said they were the most successful crusading order. Their conquered kindom lasted longer and was larger than any of the others I believe. (unless you count the reconquista as a crusade)

Anyway, they would be a challenge to Poland and Russia in the game and would not make it so easy for those nations to collect all those empty eastern provinces.

Also, if Im not mistaken, they tangled with the mongols aswell.