For the amount of vehicles that the US has developed over the years, it's been a long standing opinion of mine that the US needs more vehicles in the tech tree.
Of the tanks we don't currently have in WWII, perhaps the most famous is the M60A2. The 152mm HE launcher MBT that was a bit of a failure. Big gun though, and we all love a big gun!
The M60A2 exists in CW as an Era 2 Tier 3 medium, and it has the potential to be very powerful, armed with HE and ATGMs. It does, however, have two rather concerning traits.
Firstly, missiles. For WWII, we absolutely cannot have ATGMs.
Secondly, armour thickness. It's a chunky tank.
The first one is fixable by making it HE/HEAT only, but the second is trickier.
Enter the prototype, the M60A1E1. Unlike the production variants with their thicker frontal hull armour, the prototype was based on the earlier M60(A0), the dome turreted first production variant. This means the same hull armour as our M60 premium.
The 152mm gun is, of course, a slight balance concern. The solution to it is, in my opinion, lowering the HE penetration and focusing on HEAT at a modest penetration. I've seen prior suggestions for an unguided rocket too, something I'm not against.
We also have another couple of tanks that could lead into the M60A1E1, providing a nice branch of vehicles.