TLDR: Giving new natives and mercs to old civs would not do any harm, because most of them are just designed for fun. If they give rise to new builds that’s even better.
Since the very beginning, I feel the merc/native cards are designed based on the following considerations:
- The civ and the unit are somewhat related historically. (Example: Hessian and Cherokee for British, Huron for French, Mameluke for Ottomans)
- We need to keep the format of “having some native/merc cards for all civs”, but there is no historical basis for some civs, so let’s give them random ones (Example: Nootka for Russians, Cree for Germans, or the random native cards given to Swedes that do not even have a theme like older European civs)
- It’s a new unit added in the DLC. Let’s give it to the new civ in the same DLC. (Example: all DLCs)
These give rise to some balancing issues, undoubtedly, but that’s more of a retrospect. At design phase I don’t think native/merc cards were carefully balanced, but were rather randomly assigned, or just for fun.
For example, if I look at the Italian and Maltese native cards, I do not think Italians REALLY need an age 4 card for 25 Bourbon musketeers, or Maltese REALLY need those hand cavalry natives like totenkopf or boyars. By “need” I mean either the civ lacks the standard unit, or the civ is so weak in all other ways and has to depend on that card. That is to say, if you give them to other historically related civs, say French, British, Germans and Ottomans, that would not have a significant impact.
Some cards might make some good builds on their own of course, but mostly because these cards are good by themselves. I do not really see a useful native/merc cards that becomes useful because it synchronizes so well with the play style of the civ (unlike most unit buff or eco buff cards). If you give it to any other civ it becomes a good build.
For example, 4 mamelukes for Portuguese in age 3 is a good card. It’s not just good for Portuguese, it’s good for any other civ. Ethiopian age 2 12 dervishes used to be a toxic card, because it’s a good card for a good unit. Ethiopians have other options as well.
If a new native/merc card could potentially open up a new build order for an old civ, that’s a great thing. The biggest advantage of AOE3 is that you can actually make a new “civ” out of an old civ by changing the deck build, while the old civ with its very well established play style remains untouched. Especially at a time where very rarely are decks build around native or merc cards.
On the other hand, there are of course cases where a merc card synchronizes really well with the bonus of some civs, and thus need balancing tweaks. But I would say these are rare because we don’t see a lot of such examples with the current assignment.