Bucklers certainly were popular in warfare all be it a bit later than the period Battle Brothers takes its inspiration from. I think the trap you have to avoid is coming up with all sorts of bonuses and skills to make something useful or differentiate it from something else just for the sake of inclusion. I like Gorlums suggestions above though not so sure about the need for dome shields.
I would suggest that shields should go something like;
Buckler – low weight and low action point to deploy, decent melee block chance, rather than Shield bash knocking the opponent back you have a chance to stun – hitting out towards the face with a buckler was a common tactic.
Round Shield – as it is
Heater Shield – should be the same as the round shield but more durable and with slightly better stats – heater shields tended to be made thicker to better resist weapons blows (hence the move from centre boss to straps).
Kite Shield – I think the bonus should be to melee defense not missile. the long teardrop shape protects the legs from melee weapons but other than the fact its a little bigger I do not think it does anything more than a decent sized round shield to protect from missiles which are normally coming down in an arc or if fired directly aimed at the chest/head.
Pavises for archers/crossbowmen – should provide good missile protection but no melee protection (you can just push/pull them over or step round them).