I don't see an MP faction as all that challenging, your opponent has the same strengths and weakness as you do, the missile units have the same capability, your cavalry can charge and flank, disengage, re-engage, wheel around just as quickly as his. Your Spearmen have the same reach as his.

Where is the challenge in that? The challenge lies in trying to beat someone who is an awesome commander of the Horde with a faction that is mis-matched (HRE, Byzantines, Italians). Or using cavalry in a place that is ill-suited for cavalry (Scotland is an example)...

The true challenge would, I think, lay in what ever faction you pick versus what your opponents pick. Imagine trying to defeat the Horde with a HRE force of Gothic Sergeants and Knights? Or a Byzantine force of Kataphraktoi?