In aoe 3 I always loved to be able to recruit natives and mercenaries to go past your population and add a little more variety to the units you can recruit. If aoe 4 doesn’t get mercenaries, I will sorely miss them. I think that by adding an embassy mechanic similar to what Asia had where you can ally with a foreign mercenary companies and recruit units, but also get access to unique upgrades that are applicable. For example:
‘In the case that the English ally with a Spanish mercenary company they might open recruitment for rodeleros, tercios and arquebusiers while also providing a purchasable ‘shipments of yew’ upgrade that improves the range and power of longbowmen (archers if another civ).’
Swiss company could allow recruitment of swiss pikemen, swiss halberds (armor peircing variant of pikemen) and lucerne hammer infantry(does an area affect every 6th attack, is armour peircing, is a swiss polearm) . Swiss mercenaries have long melee range so the can benefit from being able fight from up to the third row from the edge of their formation. Charging cavalry take damage when charging swiss polearm units. An example of a unique tech could be ‘Almain rivet’ the flexible armour could increase the speed of spearmen and men at arms while also increasing their armour. ‘Swiss daggers’ could be a tech that improves the melee attack of ranged units and villagers.
These are just two examples I came up with. Mercenary companies could add alot to the game.