There are a lot of weapons already in the game that can/could be used as crew-served weapons:
AT-guns, Machine guns if used as a Heavy role like the MG-nest the engineers can build, AA-guns, Mortars.
The Engineer no longer would be able to build AT, AA or MG stuff.
There could be Weapon Crew squad that can operate all of those and the type of weapon would be selected in the lobby the similar way you can take either T-60, T-26 or BT-7 in the game. They could also have an individual squad for each, or the type of weapon could be selected in the battle before spawning, I dont have a clear idea of the ideal deployment.
For example here is a possible HMG Squad for USSR. All the guys would maybe have the 100 round ammo canister, everyone could shoot the Maxim gun but only the designated gunner is really good at it so maybe more recoil for the non-gunners if they use it. If all the guys are alive, 1 would shoot, 1 would feed the belt and reload, the other 2 are there for supplying ammo and take the other guys place if they get killed.
The Maxim could be moved around the battlefield walking speed and can be broken and repaired to certain extent. When the last one from the squad dies the gun disappears(?).
AT-guns were movable by the crew and that was essential for their good operation, especially stuff we have in Moscow campaign is lightweight enough, i dont know about 75mm guns and above in later campaigns though.
List of possible weapons for each faction:
AT:
Germans (Normandy & Berlin) - 8.8 cm Raketenwerfer 43 - Wikipedia
Allies (Normandy & Tunisia) - 37 mm gun M3 - Wikipedia
Germans (Moscow & Tunisia) - 3.7 cm Pak 36 - Wikipedia
Allies (Moscow & Berlin) - 45 mm anti-tank gun M1942 (M-42) - Wikipedia
Mortar:
Germans (Normandy & Berlin) - Kz 8 cm GrW 42 - Wikipedia
Allies (Normandy & Tunisia) - M2 4.2-inch mortar - Wikipedia
Germans (Moscow & Tunisia) - 8 cm Granatwerfer 34 - Wikipedia
USSR (Moscow & Berlin) - 82-PM-41 - Wikipedia
HMG:
Germans (Normandy & Berlin) - MG 42 - Wikipedia
Allies (Normandy & Tunisia) - M1919 Browning machine gun - Wikipedia
Germans (Moscow & Tunisia) - MG 34 - Wikipedia
USSR (Moscow & Berlin) - Maxim gun - Wikipedia
AA:
Germans (Normandy & Berlin) - 2 cm Flak 30, Flak 38 and Flakvierling 38 - Wikipedia
Allies (Normandy & Tunisia) - M2 Browning - Wikipedia (double)
Germans (Moscow & Tunisia) - 2 cm Flak 30, Flak 38 and Flakvierling 38 - Wikipedia
USSR (Moscow & Berlin) - 25 mm automatic air defense gun M1940 (72-K) - Wikipedia