Riflemen are a different breed from the standard Redcoat, so they don't stand out as uniquely Brit; they really stem from Capt. Minie's 19th century conical expanding base bullet. If you want a uniquely Brit Redcoat, do something like give it an extra HP--that more accurately characterizes the steadiness in Civ terms that we associate with the Redcoat in history.
I'd argue, though, that British seapower was even more characteristic, especially the frigate. So I'd look for a frigate improvement.
As for others, I think you could make a case for the German Stosstruppen, as this revolutionized 20th century tactics, as opposed to the Panzer (which was inferior, technically speaking, for most of the war--what the Unit is really getting at is the German revival of mobile war, but the infantry revolution is even more important, IMHO). Several ways you could go with this: higher attack/defense, more HP, an extra movement point, ignoring free shots from armies/barricades/missile troops, ability to build their own forts, or--my favorite--when they attack, cutting any defender benefits in half (or even ignoring altogether) or better, & getting rid of even the grassland bonus. Because that's really what the Stosstruppen did, made the defense much less effective.
For the US, the F15 is a headscratcher. Certainly does not reflect anything characteristic about American power. I'd want something like the Minuteman, an improved musketeer, with extra speed, maybe, or much lower cost (to reflect the ubiquity of an armed militia). Or a carrier that actually does project some power--maybe it comes with a fighter unit automatically attached, or awesome defense to reflect the CAP & escort group native to it. Or a better heavy bomber: range of, o, 20 to 30. That much more reflects US power projection in the 20th century.
Or, again my favorite, ToT (time-on-target) artillery. This refers to all artillery fire against a given target impacting at roughly the same time, to within a few seconds. this is infinitely more devastating than spreading a bombardment out. The US didn't invent it, but perfected the system before WWII, along with the capability of allowing someone like a platoon leader to call down supporting fire from darn near every tube in an entire corps, much less his own division, & to get it there on time. Not sure how you do it in game terms, but the effect would be something like a much greater probability of hitting the target & doing damage from the RNG, and/or possibly a much greater bombardment rate--give it a 4 or so.
Any of these would make more sense to me than the F15, which had a relatively limited run, vs. these items.
kk