Would it be possible to have a unique Unit Upgrade that would replace the general Unit Upgrade?

With RoR we have probably every unit upgrade that will ever be in the game with the introduction of Imperial Skirmisher. I must say I dislike it as a team bonus, just as I dislike Genitour/Condotierro as team bonus units. I prefer special units to be something exclusive to civs. I prefer the Indians' design who get an exclusive Camel upgrade as a substitute of the Knight line. However, it's not possible to develop "more Indians" easily, since there's no unit left to be upgraded further, maybe except the Scorpion which (unlike Onager and Ram) doesn't get a second upgrade. I also don't think gunpowder units or trebs would be good targets of such treatment, because of balance (trebs) and the fact that you already wait long for gunpowder units.

What if there was a way to substitute existing unit upgrades? I could see a civ that would have a different Champion or a different Halb, or even a different mid-upgrade that would upgrade to the general unit (like a unique Cavalier upgrading to a regular Paladin). Would it be a feasible way to make new civs?