There is any change to bring models billboard back? I know its seems useless but trust me, modders always find a way to use everything.
There is the thing: I did pbr for all dropable guns but, as you know, sprites don't work with pbt, Models does. The big problem is the fact that plane models needs to billboard.
I did some tricks to emulate this behaviour creating new actors that modeldef calls plane models with the gun sprite as skin. The A_FaceTarget do the magic but isn't perfect.