So the goal here is to discuss the best way to achieve the following :
- It's always a bit meh to have units not having some MCU upgrades while others do because they were produced before any MCU was unlocked. Generally speaking, you are stuck between losing a high level of experience on a unit that you had since the beginning versus losing the MCU upgrade.
- It can also be frustrating to wait to create certain units because you want to have a certain MCU unlocked first. So you delay your troop production just for that but then you become vulnerable to attacks.
- If there was a way to get back, with a decent amount of balancing limitations, newly MCU abilities unlocked to older units it would feel good for everyone- no more head scratching about the timing of troop creation, better game flow, etc.

Solutions considered :
1) Adding a "visit site effect" to all MCU structures of the game, giving permanently to visiting troops the ability that the MCU provides once it's built in the city, with some sort of weekly timer (7ish turn cooldown)
2) Similar to 1), The MCU built in the city also provides to builders produced a new ability, to be able to create some sort of training camp structure which would provide to visiting units the ability (with a turn timer as well).

[!] Issue identified- I investigated this solution, and there is one big problem: you cannot apply a requisite filter on abilites given by visiting sites. It means for example, in the case of Ancient Ruins, that all units in the stack regardless of being pikeman or not would get the "Pillar of the stylites" ability. That's a big deal obviously.

3) Adding a spell as a reward from clearing the structure which could be either a global enchantment spell or a target-army instant spell, which once casted would apply the property of the MCU to all appropriate units.

[!] The easiest to do by far, and the issue is more about the balancing implications of this- does it sound fair to be able to apply to your entire army a powerful ability just after clearing the structure? It means you neither need to have it in your domain nor to build the MCU in a city to benefit from the upgrade. It could be a bit balanced by applying a high mana cost, but still.

4) Adding a new Hero item as a reward for each MCU, which does not take away any equipment slot, which provides to all units in the army of the hero said ability.

[!] Would work too, require some investigation nonetheless.