the issue with removing the PIAT for the M1, is that the M1 would be completely unbalanced compared to the Sturmpistole, you can try it in the shooting range (along with the Thompson M1928), it has no recoil and a much more forgiving ballistic arc (the round is not much faster, but is much lighter) making it easier to use compared to the broken arch vault of the PIAT.
you may get it later with an expansion of the campaign, but it will only come with the German getting also either the Panzerfaust or the Panzershreck
the secondary issue is that the US doesn’t have a direct equivalent to the Strumpistole (which is essence, a flare gun with a stock), the closest is the rifle grenade adapters for the 1903 (M1), P17 (M2) and M1 (M7) rifles that allow to fire the M9 AT grenade (among others) but then that pose the issue as to how to introduce either to the game in lieu of the PIAT (currently rifle grenades are attached to a special variation of one of the nation’s rifle and is usable by everyone).
i have nothing against picking up spare ammo from fallen comrades, as currently it’s very easy to run out of ammo (esp if you have a rifle) and replenishing is dependent on either having a soldier with an ammo pouch in the squad (praying that he wasn’t deleted from standing around like a gopher) or an engineer building an ammo crate not too far.
as for canister shot, it was used, issued to tanks armed with 37mm M5/M6 canons the issue is again that it would have the risk of being too powerful, with no counterpart from the opposing side as Germany didn’t use canister shot with it’s tanks, currently tanks are deadly enough against infantry, no need to give them extra tools to mow them down, if they need to have the anti dough capabilities increased, maybe buff the HE a bit or slow the MG heating up, also with the current way to change ammo, introducing an extra ammo type would induce issues.
the Thompson may come later as a replacement of the STEN or Lanchester (both?) so that is not an issue for now