Two suggestions, both regarding buildings.

Valour bonuses.
That is, valour bonuses which come from buildings. I'm not sure how readily these can be changed by modders and if they could, what would be worth changing. However I wish to raise the point that in the Viking Campaign buildings which gave a valour bonus to a particular unit, such as the lvl2 archer building for archers, said in their description valour + 1. The church/shrine said morale + 2 (or whatever). This feature is missing from the medieval campaign and that annoys me. Even now I am unsure which buildings actually increase the valour of corresponding units. I think the VC descriptions were a step in the right direction, although even they only said "Valour + 1" not the unit the valour upgrade was for. This addition in the VC appears to be hardcoded to me, rather than simply being an altered description, however what I propose is this:

The buildings which give bonuses have their prose description slightly altered to make it CLEAR what kind of unit benefits with a bonus and then have the bonus itself laid out CLEARLY in BLOCK CAPITALS at the end of the description, like a technical addition. I don't think this would hurt the immersion into the game to any great extent and it would help out a lot of players.

Examples.

"The hyperblacksmithsupermetalarmourer employs new metal making techniques to blah blah blah and all military forces created in the province benefit from it's presence.

ARMOUR + 3"

"The church of the divine arsebucket blah blah blah leaving all military forces from this province with a boost to morale from their faith.

MORALE + 2"

"The military training hall is a place for veterans to pass on their knowledge blah blah blah allowing the recruitment of elite troops and giving all lesser sword infantry a bonus to valour.

VALOUR + 1"

Sorry if I'm overexplaining, just making a point about clarity. It's only mildly annoying and can be very useful.

Mongol build tree
I'm not sure how you are planning to deal with the Mongols Wes, but one thing I would like to see is their build tree simplified by barring them from constructing buildings which are of no use to them, since they have a limited selection of units with very similar (perhaps should be made even simpler and similar) build requirements.

That's all for now.

EEUURAAH.