Altho more mortars are nice there already is an doctrinal indirect fire unit in Mobile Assault so it doesn't make sense with the overlap there, same thing with the Ost and Soviet commanders that have the Mortar HT/HM-38 and their respective artillery piece and I've almost never seen people use both.
So just an idea but maybe throw in some sort of vehicle in there like a Halftrack or a tank, something like a Valentine/Sherman or AVRE Churchill?
A tank with an infantry support role basically is my thought, there was even a "Close Support" version of the Churchill armed with a 95mm howitzer that somewhat looks very similar to the AVRE but would act more like a Stupa/StuG III E/105 more than a point and click barrage ability.
Pic for reference:
It was called the Churchill Mark V I believe.
If it were a support vehicle for mobile assault, then instead of the Churchill, i think the Centaur iv close support will be better fit. As mobile assault regiment have commando and centaur cs were use by royal marine armour support group and will be more mobile, it is basically a cromwell with the same 95mm howitzer.
On the other hand, such new vehicle is less likely to make it into the game, to be practical i will create a Supplies universal carrier which able to reinforce and heal, like an ambulance clone but a bit morr durable to work close to the front line, pretty fit mobile assault theme.