He-115 is available as a frankenplane at IL2 Free Modding, but its not very good.
Champi is working on a BV-138. The model looks lovely, but I don't know the status of the project.
As for the rest:
BV-139, BV-222, BV-238, Ha-140, Do-26, FW-62, He-114: Prototypes only or very limited production and use. There are whole air forces of more important planes yet to be modeled.
Do-18: Obsolete by the time the war started, quickly relegated to training roles. It does have the dubious distinction being the first German aircraft shot down by the British, however. (3 Do-18 vs. 3 Skuas. Result: 1 Do-18 damaged and forced to make an emergency landing. That tells you something about the operational effectiveness of the Skua!)
Do-24: Produced in some numbers. Used by both sides during WW2 (Germans in Europe, KNIL and later Australia in the Pacific). Also used by France, Spain and Sweden. Possible candidate for modeling if there's enough data.
German land vehicles are well represented in the game. About the only weak spot is various self-propelled artillery guns like the Wespe and Hummel and perhaps a few more river-crossing vehicles (assault rafts, schwimmwagen, armed barges).
Relative lack of artillery isn't that much of a problem, since most towed howitzers look pretty much alike from an airplane. It wouldn't hurt to have an infantry gun or mountain howitzer, though. And, other than vehicle-mounted rockets, every country is short on self-propelled artillery guns.
Ships are a weak point in the game. It would be nice to see more small ships, which saw more action than the big ships during WW2: submarines, patrol boats, frigates, destroyer escorts, destroyers, minesweepers, minelayers and armed trawlers. I wouldn't complain if someone modeled a German cruiser, though.