Wouldn't it be epic if Rus, the Holy Roman Empire and the Mongolians all went to war with one another? It'd be the great war of all time in the let's play.

Let's not forget the Byzantines, Random Muslim nations, the revolting Cumans, England, France. ETC.
Basically if that happened then the world would be a firestorm of x Nation declares war on y nation then z nation jumps on the opportunity.

which would even be more epic. It would really be a bang to go out like that. Yeah have those three countries and their allies go to war with one another it'd be the mother of all wars.

What if Rus and the Mongolians teamed up the king of Rus marrys the Mongolian princess or Queen and gains all the that territory through succsession

Lot's of pissed off vassals.

I dont' think the mongolians would ever agree to marrying into the locals.

My friend the entire mongol culure was spread by not just conquest but also intermingleing with local populice from China to Russia and every thing in between. Kings and nobles of the far east would would even try to legitimize there claims by claiming derect decent from Ghengis Khan himself. Why not take full control of the horde and Russia. Stalin has already inter married to unite a empire why not again.