I would like to suggest the KD-I Geschützkraftwagen 19 mit 7,7 cm Flak 16 L/35 one of quite a lot of WW1 SPAAG
The history behind it is a long path, starting in WW1 in 1918 going till somewhere befor or even in WW2.
In 1918 on basis of the KD-1 Artilleriezugmaschine (Artillery tractor) (from 1917 to 1918 were 1.159 produced) was the KD-I Geschützkraftwagen 19 build by equipping it with the 7,7 cm Flak L/27 (Used at that time the same ammo as the 7,7 cm FK 96 L/27 with about 465m/s) which was made in the beginning to mobilize the 7,7 cm Flak L/27 for anti aircraft and anti balloon usage, beeing able to escape enemy artillery fire, while also beeing effectively used as an mobile anti tank platform beeing able to quickly react to enemy attacks of Tanks. The Vehicle was regarded highly by the Reichswehr for its high usability as AA and AT vehicle, beeing even kept after the War.
Overall not just the L/27 guns were used, the ammo of 7,7 cm was chosen so enemy captured equipment such as french 75 and russian 76,2 mm guns could and were drilled out to accept german ammo, which even a wide range of different guns were mounted on these vehicles from L/27 to L/35.
As well as the more powerfull Fk 16 L/35, which is mainly just a longer barrle with 545m/s instead of “just” 465m/s.
Overall there were quite a lot of different guns and mountings used on them such as as above mentioned drilled out french and russian guns, but also going further than simple Sockellafetten going as far as mounting shields and even big turret arrays, which in this case i suggest the L/35 with gunshield.
Why i would like to see this vehicle? Old stuff can take a grip in game, but is still underrepresented and this is one of the vehicles which are actually quite good and will be quite usable (notice characteristics of the truck and gun below) Bringing an quite good Gun on a mobile chassis with similar playstyle ot the italian 100mm gun, but with greater effeciveness, otherwise the gun (essencially a 7,7 cm FK 16 L/35) would also have no place in game as it was not mounted on other vehicles to my knowlege while it is also historically relevant in beeing the starting point of the stronger AT weapons for the german army in WW1, beeing also used as close combat AT guns.
The Gun: 7,7 cm Flak 16 L/35 (a 7,7 cm FK L/35 in a Flak Sockellafette)
7,7 cm about 12-15 rpm -5 °(or -8°) to +75° and 360° traverse
60 rounds of ammo across different containers around the flatbed and in the vehicle sides.
Ammo: There are 5 rounds
K.Gr. 15 m.P. Saphe 6,85 kg 230g Amatol 545m/s with 64mm/10m (has no tracer)
K.Gr. 15 m.P. m.L. Saphe-T 7 kg 230g Amatol 545m/s with 65mm/10m (has a tracer)
And 3 He shells:
K.Gr. 15 He 7,1 kg 380 g Amatol or 270 g Picric acid
L.F.K.Gr. He 7,18 kg 930g amatol or 700g Pieric acid( ig x1,1=770g, IRL x1,2=840g) or 820g amatol with 75g Pieric acid. with 545m/s
C-Geschoß He 6,09 kg 550-600g Amatol 602m/s (streamlined boat tail long range shell also usefull for AA)
Vehicle: Artillerie-Kraftzugmaschine Krupp-Daimler 100 PS
Weight: 8000 kg
Length: 6265 mm
Width: 2170 mm
Height: 2860 mm
Axle articulation: 30°
Engine: 4 or 6 cylinder 100 HP/74kw engine
Speed: 8 gears forward with 65 kmph (conflicting sources, going from 36 kmph with 10t attached load to 65 kmph)
3 reverse gears to 8,5 kmph
Ground clearance: 400mm
Turning radius: 6,5m
Crew: for this vehicle in game somewhere between 6-10
Armor: around the Hull 3-4mm (wow so thick, will deflect some small he rounds and rifle bullets from afar, nothing more)
Gun shield: 14,5 mm
I am sorry for errors and slight missalingements, its just so old.
Notes on German Shells, 2nd Edition
Geschoßringbuch Band I
Munition der FK 16
Das Gerät der Artillerie vor, in und nach dem Weltkrieg
Encyclopaedia of the German Army in the 20th Century
Vom Einzelschuss zur Feuerwalze: der Wettlauf zwischen Technik und Taktik im Ersten Weltkrieg
Deutsche Artillerie 1934-1945
Weapons of the Third Reich An Encyclopedic Survey of All Small arms, Artillery and Special Weapons of the German Land Forces 1939-1945
German Explosive Ordanance Projectiles and Projectile Fuzes Chapter 5