o.k firstly seppaku is not done by samurais or ninjas as it is chinese you are thinking of hare kari (slightly differant)
IMHO a game should be either european or asian not both as it's unbelievable and stuff. It'd be cool barehanded fighters from japan-karate and judo, from korea twae kwon doe (against mounted opponents), from china kung fu (also includes weapons and has like 12 differant sub classes). With weapons you have samurais and ninjas who also use martail arts. Theres Mongols by the way the mongols did take big cities, they sorta took constantinople (the biggest city of the time and is now instanbul). There also could be the ancestors of the cossacks if you want to stretch it for even more horsemen. There are many othes to boot.
Magic could be brought in with differant forms; ninjitsu, chi (mainly defense magic) horse magic, meditational magic etc etc.