With the Definitive edition every civilisation got their own crest in the interface and it’s also used on some of the unique units of those civilisations.
It would add a lot of flavour to each civilisation if generic units like Long Swordsman, Skirmisher or Knight would use those on their unit sprites similar to how every building set also has unique sails for the ships.
Depending on the shape of the crest and the boni of civilisation the crest would be used on different units. If the crest doesn’t fit a unit the generic one is used instead. There could also be generic shields for each architecture set.
Always keeping the shape of the original shields to make the sprites fit on easily for every angle and keep the readability of the units.
(simple edit without proper shading. The crests on the castle would need some more work but it doesn’t move so it wouldn’t be too hard.)
The Frankish, Teutonic and Burgundian shields would look perfect on a Knight/Cavalier or Paladin.
The Aztec or Vietnamese skirmisher would look nice with the round crest of those civilisations.
While the round shield of the Saracens, Turks, Persians, Malians, Tatars or even Chinese would look nice on a Camel Rider.
Those could also be applied as decals on buildings like the Town Centre, University or Castle of some civilisations.
This would be a relative easy was to make units look more special while still being very readable.
Seeing a Paladin with a Frankish shield you know it’s gonna have more HP or a Camel Rider with a Malian one will have a high attack.
A similar system is already used for ships so it should be possible in the engine.