I'm all for a diverse array of vehicles and weapons to be added to the game.
That being said, they all ought to have a specific role or, at least, a specific timing with which they are strong and useful. A panzer III could easily be a slightly worse p4 and force a player to choose between a weaker tank now or a stronger tank later.
I'm also of the opinion that coh3 could be some kind of "ultimate" coh experience; a separate tuning pack and direct port of old factions could be worthwhile in certain cases (Panzer Elite, Soviets, OKW?) I'd also like to see some early war stuff. I'd love to see one or more factions that uses Dutch, Belgian, French, and British equipment/soldiers. You solve the tank technology issue by having different periods of action where the late war german faction cannot (at least in automatch) compete with the early war French faction.
I always thought the best thing for it to have a role would be a diverse amount of upgrades. Panzer III could start with short 75mm or older 50mm cannon. From there it could be upgraded with later war 50mm, side skirts, or maybe permanent APCR. An upgraded Panzer III could be roughly equal to a Pz. IV at the end of the day.