Just read through the Unit Portrait thread and someone stated about Clone Wars got stuck in my minds.
To the average RTS player, clone wars are an inevitable byproduct of any kind of unit spam, and it can make things look repetitive.
In Medieval II: Total War, they introduced an system where all the units have variations cosmetically to avoid such a problem, like height, design on shields, etc.
I will advocate this system if possible to the game. Different markings, like numbers on the tank, special paint jobs to indicate veteran status, even equipment on the infantry, like the shielding, etc.
Another more radical approach could be how the tanks used by some factions can be varied cosmetically, like how perhaps the Allied light tanks can include a few model variants that function the same, but have different looks. This does make things more complex to code, but you do get different equipment that does the same job through the course of the war, cosmetic or otherwise, and such variety can add life to a battlefield IMO.
EDIT: Here's a repost to the reply I gave in the factions idea in Generals 2.
An suggestion by AGM, as he pointed out I have overlooked upgrades, and I agree with him, so any sort of customization and additional rendering cannot, and should not affect how one can visually see an upgrade, like the railguns in C&C3.