One of my favorite things to do in FF is to go into the World Builder and add in units from other civilizations to my armies, like having some elven archers join my human armies, just as a kind-of role-playing thing. Because I always play single player, it really doesn't matter that I'm going into the World Builder to do this, but others might like this option without cheating in multi-player.
What might allow this is a National Wonder for each of the other civilizations active in the game, i.e., all opponents in a game, called an "Embassy". This wonder can be built by any civilization that has open borders with the civ in question. Once built, the owning civilization could build one or more different unit types in the style of the other civilization (not necessarily Civ-Unique Units, but definitely with traits common to the other civ, or have them be Limited Quantity only). The Embassy does act as a viewing point for the other civ, like common religions or espionage, meaning you may want to build it in a non-key city. Having an Embassy would improve the relations between the two civs, and a civ can have as many Embassies as other civs with Open Borders to them.
If the other civ breaks off relations, e.g., dissolves treaty, goes to war, is defeated, then that Embassy goes dark (including their view of your city) until Open Borders are re-established (if possible). Not sure what else you could use the Embassy for, but it might offer a culture bonus, great person bonus (adventurer for non-Grigori's?), etc. Might be better suited to a larger Espionage effort, and then be involved in that process much more.
Again, I'm not a modder, so I throw this suggestion out to those looking for a cause to champion.
What might allow this is a National Wonder for each of the other civilizations active in the game, i.e., all opponents in a game, called an "Embassy". This wonder can be built by any civilization that has open borders with the civ in question. Once built, the owning civilization could build one or more different unit types in the style of the other civilization (not necessarily Civ-Unique Units, but definitely with traits common to the other civ, or have them be Limited Quantity only). The Embassy does act as a viewing point for the other civ, like common religions or espionage, meaning you may want to build it in a non-key city. Having an Embassy would improve the relations between the two civs, and a civ can have as many Embassies as other civs with Open Borders to them.
If the other civ breaks off relations, e.g., dissolves treaty, goes to war, is defeated, then that Embassy goes dark (including their view of your city) until Open Borders are re-established (if possible). Not sure what else you could use the Embassy for, but it might offer a culture bonus, great person bonus (adventurer for non-Grigori's?), etc. Might be better suited to a larger Espionage effort, and then be involved in that process much more.
Again, I'm not a modder, so I throw this suggestion out to those looking for a cause to champion.