The thing about the M18 is its firepower/speed combo would be unmatched. It would truly offer something in the GV realm that is currently not in AH.
Some other points about the M18 that does not get brought up:
It has no hull MG, so if the turret gets knocked out, then it is truly helpless. Its .50cal HMG on the pintle mount does have an abundant amount of ammo (supposedly 800 rounds).
The M18 and its M1A1 cannon (same as on the current M4A3 76mm tank), carry only 45 rounds of ammo, and most of that would be AP. If AH followed typical US Army TD unit doctrine, very few HE rounds were carried. In AH, the Sherman VC "Firefly", the game's only designated TD, is able to go full HE if the players chooses and that really shouldn't be, imo. With "only" 45 rounds, players will be calling for gv supplies rather quickly.
The M18 had at most 1 inch (25.4mm) worth of armor up front, otherwise it varied all the way down to 5mm. Even the M4A3 and T34/76 w standard AP would have an easy time destroying the M18 IF they connect.
With an open turret, the M18 would be rather bent-over and cheeks-spread exposed to enemy aircraft gun fire. Even the 100 or 250 lb bombs would be a menace. It would be easier to turret than the M8.
With all that being said, it would be MUCH easier (quicker implementation from idea to addition) for HTC to add in the M10 since the chassis is already present in the M4 (the M4A1 chassis is onlymarginally different than the M4A3), and only the gun (M7 3in) would need to be coded as we currently do not have it in AH.
