Yep, would definitely be sweet. Having AT guns would be fun on a map with a few parallel 'alleys' where attacking tanks were sure to move into, and the AT guns would spawn randomly at a couple of those alleys, meaning the enemy won't always know where it'll be.
I even thought of a system I liked where instead of "mounting" the gun like you do with machine guns, you simply walk up to the different controls to operate them individually. Loading would be done by walking up to the ammo pile and then to the breach of the gun. That would allow for single users to operate as well as giving realistic incentive for teamwork, all the while not adding any extra animating or coding for multi-player crews.
The problem with medium and heavy mortars (the 8 cm granatwerfer and the 120 mm Soviet mortars) is that I assume they'll be stationary like the hmg's, so it will take a lot of vocal coordination with your team to be effective, something I fear will be lacking in public servers. Apparently people made it work in DH, but to what extent this would apply here I can't say.
The other thing I'd find weird, though which isn't a flaw in itself, is that there's already a mortar call-in, and something about having those two being the same weapon doesn't sit right in my noggin. But that's probably just me being pedantic.