The flaw with your argument is that Firaxis has already chosen Frigates to represent other nations' "Ship of the Line". Like the so-called Lancer gap, this is a non-issue. We could simply change the name to Man o' War, and there you go..."unique".
The bigger problem with so-called "uniques" is that none of them are actually unique beyond uniforms or very cosmetic differences. Saying that England shouldn't have Ship of the Line as their UU is a slippery slope that ends with an argument for eliminating everything unique from the game because none of the uniques are functionally different from the things they replace, IRL, and are just as easily represented with a generic...
...and four uniques is a bit overkill and also runs the danger of over-conglomerating cultures, e.g., Rome and Italy who couldn't be more culturally different, or rolling the Sumerians into the Akkadian Empire and its ingame successors (Assysria and Babylon) or rolling the Hittites and Phoenicians together. The other problem is what effects are you going to use to make these extra uniques distinct. There is a limited pallet of mechanics to choose from and IMO, all of the interesting ones are already in use, i.e., this was the purpose of uniques in the first place, to riff on the rules and make civilations different from one another through more personalized means than technology and social policies since you can't really express cultural differences any other way in an abstract game like CiV anyway.
IMO, I'd prefer to see regionally distinct architecture and unit sprites before we see fourth uniques.
YMMV