Should mercenaries be an option to introduce new units or this would be to weird for aoe2?
You could make a new building like a tavern or mercenary hub to recruit them and make a shared pool for all the faction, so if one recruits a company, all other players can’t until they are replenish.
Mercenaries could be more expensive than regular units and in some cases stronger (of course not stronger than paladins or CA), they could be recruited very fast and only in groups or companies, so it could be a expensive way to defend yourself from a rush or to go over the population limit (like feitorias)
They should not be upgraded and new companies would be available in every age. Maybe some civs could have bonuses to reduce recruitment cost or cooldowns
As a complex mechanic like stated no, but a ‘mercenary’ type unique unit that could be trained exclusively with a (high)gold cost at castles wouldn’t be too extreme for a future addition to me (Either as part of a new civ or added to an existing civ as a second unique unit) - as long as they aren’t taking over the role of the civ’s current UU
Several of your examples would fit existing civs thematically, Varagian guards fit Byzantines, Swiss Pikemen could fit a number of civs etc., if they get the cost and strength of units correct they could add them without trashing balance (Decent enough to be used sometimes but not cost effective for most situations) - maybe with very excessive cost but fast train time
That could work too, maybe some of those mercenaries could be shared by many civs, although i think they should not be created from castles but from a new building that only cost wood.
Also i think they should not be upgradable to an “elite” version, they should be expensive powerfull units with a very short training time. They also could cost no pop (but have another kind of popultion cap or limit), that way you could have an edge and be “persuated” to expend the extra gold in those units
i like ur thinking and i think that the mercenaries can be from all over the world and maybe have the same 4 or 5 mercenary tipes ( cav, archer, infantry, pike and light cav) and they are diffrent from place 2 place. For example if ur playing arabia u have desert archer, camel rider , arabian pikeman, atc and if ur playing yucatan u have eagle worriors, some kind of archers and maybe priest warrior (monks which can convert, heal and fight) and if ur playing black forest u have germanic warriors and heavy germanic cavalery. I htink this will break the meta but at least some 4 or 5 maps should have this and i rlly like the idea
Mercenaries where only common at the end of the Middle Ages so making it an Imperial only feature could work.
But generally hard to balance. They should not be affected by any civilisation bonus or technology. So no armour upgrades, no Chemistry etc. And I think they should cost population.
But than I don’t think they would add to much to the game unless the system is expanded and every civilisation is given a unit that does bonus damage against mercenaries.