While I would personally enjoy having a religious presence from a realism perspective, I fear it would be a mistake. If it were to be reflective of the real middle ages, the religions that would be portrayed would be Christianity and Islam, plus the occasional heretics on both side. Two huge problems with this would be how each religion is portrayed and the opportunity for this portrayal and contemporary reality to cause endless 'upset'. Given that Taleworlds is located in Turkey any introduction of religion into this product might actually be dangerous for them, which I'm sure was part of their decision process in the first place. So I for one, realism fan that I am, am content with a wonderful game only slightly marred by a need to acknowledge contemporary craziness.
One other mod is talking about introducing shamanism which might be fun, if only the mod had all the features of Floris. Something like that, not the major magical stuff all over the internet, but sort of hedge magic with minor potions might be fun (nothing game changing, but atmospheric), and realistic.