First, I think the Firefly could get an updated ammo load. I know there were HE rounds available, but something tells me since they were designated TD's they have far fewer HE rounds on hand than AP rounds. Why would a Firefly, a designated TD, ever go into the field with only HE??? Just a hunch, though.
Secondly, if there is a TD that could be added to the game with relative ease it would be the M10. AH already has the chassis and the gun in the game. The open turret is all that would be needed.
Thirdly, I'd vote for the M18 or Su100. Both would brig something new to AH. I hope the Su100 wold open the door for true tread traversing instead of car-like steering, but I know there are "coading" issues involved. The 100mm D-10S APHE round could penetrate 150mm of 0 degree armor at 1000 yards. The Su100 had 75mm of sloped armor up front, had 45mm on sides and rear, and 20mm on the top. Speed was 30mph at best. The main gun could only traverse 10 degrees left and right, and it carried only 34 rounds of ammo. It has the same chassis as the T34. It is void of any MG's for self defense.
