All the talk of making a German infantry UU is misplaced, imo. A UU is an example of a unit which was considerably better than their counterparts in other empire/nation's armies at the time, and each civ gets exactly one of them. Choosing the Panzer makes the most sense, because the difference between the armored units of Germany and those of every other country was truly massive. Were German infantry better on a man for man basis than any of the allies'? Sure. But the difference was not as great.
I agree with Vonreuter: The Infantry UU which makes sense is to make a Soviet Conscript UU that is available for the
cost of a rifleman, and can therefore be drafted for a single population point. (Maybe make the cost discount dependent on being in SP if that proves to powerful in game play) No other nation on Earth could have fielded so many infantry grunts as did the Soviet Union.
Hell, I'd say splitting Stalin off into a new "Soviet Union" civ actually makes sense, as would giving that civ the Research Institute. If I weren't too lazy to learn to write mods, I'd bother doing this.