As I always do, I think of how X would fit in the AH realm (MA and SEA) and not what I or another player "wishes" for. True, this is a "wish" forum but sometimes people try peein' in to a soup can on a windy day, too.
Filling the gaps has always been something I've vouched for. I've was vocal on the He111, Ki-43 Oscar, and a level Soviet bomber. Same goes for the M18 (it provided a fast platform, powerful gun, but weak armor. Something AH did not have at the time).
In terms of plane set, the biggest gaps now are now the Wellington, Beaufighter, Pe-2, Ki-100, and another German bomber. They all were important participants in WWII and they all would see use in the MA.
IN terms of GV's, the more notable vehicles that are missing in AH that would be useable in the MA are, in my opinion: StuG III/IV, Panzer III (parent chassis of StuG III), Su-100, and perhaps the Cromwell. Once HTC adds multiple Panzer III variants THEN they can add in the EW/MW British tanks with the 2 Pdr and 6 Pdr main guns. The Crusader and Valentine were huge players for the British, but stacked up against the current crop of tanks in AH they wouldn't be much of a contender. Unless, that is, if HTC allowed for the use of the HV AP ammo the British used in the 6 Pdr, then they could compete with the M4's and Panzer IV's.