One way to introduce some of the mechanics in a subtle way would be through changing cards to enable new mechanics for old units.
Tercio Tactics
This should grant the deflection ability to Spanish Pikemen instead of transforming them into Rodelaros.
Tercios were pike and shot formations, and the Pikemen in them were to defend the “shot” part of the army. This would also pair well with Spain’s Imperial level Pikemen.
Yeomen
This should give Longbowmen promotions instead of extra range.
Longbows took a lot of training to master, and a promotion mechanic would reflect that. It would also make more sense for an archaic archer not to outrange every rifle unit in the game.
The British could also follow the Italian template of having their skirmisher unit available in age 4 and a unique archer to hold the line until then. Rangers should be buffed and made available by default in age 4, and Longbows should be toned down by removing their Imperial upgrade and extra range from Yeomen. Longbows could still reach a similar potential with full promotions, but they wouldn’t be as strong freshly trained.
Counting Coup
The Lakota Warchief should have promotions enabled with this card.
Each feather in a war bonnet was earned through an act of courage and honour. This could be portrayed as a promotion mechanic.
The other main cards that buff Heros could also grant promotions. Explorers and Warchiefs are pretty tanky and cheap to revive so gating some of their strength behind combat experience could keep them in check.
Ironclad
This should have the deflection ability by default. It is already part of naval combat with the Italian card that gives Battleships the deflect ability.
Jivaro Blowgunner
This isn’t a unit in the game, but if Jivaro ever get added, promotions are something they should have. They believed that they accumulated power from the souls of animals and people that they killed.