Redesigning the M4 so that ammo isn't stowed in the bins over the tracks would solve most problems with "overcooking".
But anyway, at its introduction in 1942 the M4 was an excellent tank, but by 1943 it had lacked behind and increasingly became a burden on the confidence of allied crews on their materiel. Not only because of the ammo-fire problem, but also because of the 75mm soon becomming insufficient as an anti-tank gun. That the 76mm, which was hoped so much of, was a disappointment, didn't help matters, and it was a poor comfort that enemy tanks were the responsibility of tank destroyers, if no tank destroyers were around (they were often deployed as infantry support, when no tanks were around).
The Americans had designs with much better protection and firepower, but because of the TD doctrine and a logistic preference for lighter designs, the M4 with the 75mm was retained years after it should have been abandoned. IMHO it was a wrong decision and reflects the sometimes surprising insensitivity to casualties among early war US generals. Sure USA had a large manpower pool, but the young servicemen rightly expected to get home alive and their leaders to do everything possible to ensure that. Better to have one tank survive three battles than two tanks shot away in three battles and crewmen lost each time.
Thank God the allies superiority was so big, that shaken confidence among the tankers did not change the final outcome, but I can't help thinking if better tanks would have changed the outcome of the assaults on Caen or Market Garden in 44 - and thus have shortened the war considerably?
Concerning design details I don't think the Christie suspension was that ideal - it took up valuable space in the sides - limiting turret ring (and thus gun size). The external suspension wasn't that bad, as it didn't take up space in the sides or bottom (torsion bars) and even gave some additional side protection. A lower profile would probably in itself have removed high ammo storage - at the price of less ammo stowage - but that would IMHO have been worth it. Especially if the 76mm, and later the 90mm, had been accepted earlier.
Regards
Steffen Redbeard