New question:
Is there already an implemented mechanic for "unit with a specific promotion is the prerequisite for something", be it a building OR a unit?
There's a Wonder with a requirement mechanic for a minimum unit level, but I'm asking about actual promotions by name.
Why?
Because, and I feel like it had been discussed in the past, I'm ruminating on the idea of "promotions leading to upgrades", be it as "another way", or ultimately as "the only way".
Basically, you can only train "grunts" that are essentially Brutes, and then you must promote them in order to actually upgrade the to better units.
This, by the way, may be a "sister concept" to the "multi-type merging", but this type via "multi-promotion upgrading", probably via subcombat types to begin with.
Basically, you promote your grunt with Swordsman AND with Archer, so you can turn it into a Sword-Archer, lol.
Either by replacing with a template (but this does lead to the problem of requiring hundreds of such templates), or merely adding the appropriate subcombats without changing its graphics.
The latter, of course, would be "problematic for the human player to easily deduce visually", so we again bump into the circular problem of "graphics and ideas, ideas and graphics".
Sigh...