For the self-propelled AAA in the game:
I think the Ostwind and Wirbelwind are great.
However, the M16MGMC - while decent in the right situation - could use some love.
First off, it has 4,000 rounds... only 800 more than the Wirb, and the combined fire MIGHT be as effective as one of the Wirb's 4 guns, assuming you are hitting at a range where the kinetic energy is still effective on the .50s.
Every source that I have been able to find, none of which I would exactly call definitive, shows the M16MGMC variant of the M3 as being loaded with 5,000 rounds of ammunition.
While even that bump would be a step in the right direction, 8,000 or 12,000 rounds would be even better.
Before you say "That's gamey!" (which would be correct), consider that the real-world counterpart of the Wirbelwind was fed via ammunition box magazines that had to be reloaded on each gun, individually - although a full crew did have 2 gunners, one manning each pair left and right. (The Ostiwind could - with a decent a-gunner - maintain a constant rate of fire until the barrel melted or the ammo within reach dried up).
Now, the M16MGMC was also fed via boxes, but they contained far more rounds than an individual 20mm Flak 38 clip (I believe 200:20 or 800:80).
I know there is no reload mechanism/delay for (it would seem) guns with a bore size of less than 50mm, and I am not at all asking for the Wirb to be gimped, rather that the M16 get a bit of love to make it more competitive. As it stands, it has the worst K/D of all the SPAAG in the game (it's even worse than the C.202 this tour, in fact), and is used far less than either of the other two options.
T28E1 or M15CGMC
That we have "enough" SPAAG in game is probably true, but with the pre-existing M3 chassis as a foundation, perhaps enough merit could be found in adding one/both of the above when time permits?
The T28E1 was used in 1942, making it available far earlier than either of the German options in game currently.
The 2x .50's and 1x 37mm would be effective as well, though the T28 would get crippled by everything down to a bailed pilot (though the M15 was armored).
It will never touch the Wirb as far as sheer killing power, but it would round out the Allied SPAAG options.