I don't know about those many armour slots, but I like the idea of surcoats and faction colours.
That's something I feel missing myself. They would give a sense of unity and purpose; especially customisable colours and heraldic devices.
-> Miruim
About "stackable" armour pieces, the name you are looking for is probably "Padding".
Wearing padding under metal armour prevented chafing and cuts.
It would make sense with most chain/lamellar armour types.
We could have chain over padding, chain over leather, lamellar over hides, and so on.
I can think of about three to four stack levels:
Underpadding, Hauberk, Overpadding and/or Coat of Plates, and Surcoat.
An older game, Black Crypt, did offer that feature.
Cloaks would be nice, as long as they can flow realistically, and they won't clip into horses/bodies too much.
Personally, I would leave plate armours off.
I think M&B portrays an earlier setting, and plate armour doesn't seem that fitting, to me.
Plus, plate armour might entail firearms, and earlier armour types would seem out of place [not only historically; also for practical reasons].