Mind you I'm only posting the idea - I lack the means to bring this to reality.
A mod set concurrent with the default Red Alert 3 campaigns, with a difference: The Commander, as he is in RA3 itself, is not the player. The player takes the role of a much less significant person, without access to the cool high-tech stuff of the RA3 player. Instead of Hammer and Apocalypse tanks he has to make due with old Red Alert 1-style Heavies and Mammoths, and instead of Guardians, RA2 Grizzlies and Prism Tanks remade with Spectrum weapons.
I guess a good name for this would be Red Alert 3: The Reserves, just to establish to the player that they'll be working with some dated stuff - because that's all that's coming into play for them.