New version uploaded with another MODCOMP added to FfH which I absolutely ADORE!
Dom Pedro's Dynamic Unique Units allows you to assign Unique Units to a Player mid-game. The current fields which can make these additions are attached to Technologies (First to Discover only), Buildings & Events. More can be added in the future fairly easily (like on Civics & Religions. Though those ones are a tad trickier as you will want to remove the UUs upon changing out of that Religion or Civic. But that is easily done as long as you are careful to remember not to remove a UU for the same unitclass that WASN'T granted by the Religion).
Actually, if you tied it to religions then you could make all priests be the same unitclass, and then changing your religion would just change your priests to the new religion's priests automatically. Could be cool to do it that way indeed...
You can customize how the Unique Units will behave in the Global Defines a little bit, but overall these customizations aren't too fitting for FfH IMO.
You can limit the total number of UUs which a Civilization can have (which won't work well for FfH since everybody has a UU version of every other Hero in the game to block it).
You can limit the number of UUs which can be gained by being the first to research a tech (well, this one is a bit fitting for FfH still, it helps you to keep a person with a demanding Tech Lead in check).
You can decide if units which are already built/existing are changed to the new unit type when you unlock it. (For now I have it set so that they are. But if you did the stated example of making all priests be the same unitclass, having this set means you cannot save priests of a different religion when you swap over, which I know some people are fond of doing).
There is a LOT you can do with these 3 fields really. The sample I included with the completely new units is just a small taste really. You can reward people for being the first to a Tech with a UU version of the unit that is normally unlocked there (like first to BronzeWorking gets Axemen which cost 1/2 as much

, or have 1 more

), or you can penalize people who are too advanced in one Tech Line (like first to Sorcery gets a UU version of Axemen who have 1 LESS

, so that they are crippled in Melee if they didn't advance beyond Axemen in preperation).
You can also be creative with this: Like the first one to Research Guilds gets a UU version of Horsemen which can upgrade to an Adept. I don't know a good way to justify that one... I just thought it was a funny example
Also you can use this kind of like Hero Units. Instead of gaining Baron Duin with Commune with Nature, you could instead have them gain the ability to flat out build Werewolves (well, you could even do both if you really want to). If you want to allow ALL players access to building them, you can make the Grove building unlock access to Werewolves. Then as soon as you build your first Grove (or capture one) you can begin making Werewolves directly with

instead of having to spawn them with other Werewolves.
Remember though that as it is written, a new UU can only be gained if the Civilization currently has NO UU FOR THAT SLOT. Thus if you create a UU version of the Warrior, the Doviello & Calabim cannot get it, because they already have Beastmen & Bloodpets. A UU version of the Adept wouldn't be available for the Infernals, because they already have the Imps (and so on.. I think you get the point easily enough).