I think you already said this but these are my $.02 on it. ^^
I agree that fully customizable units is not a good thing. unless the four core classes remain the same but you can use different variations of equipment within the four core classes.
Such as more nades for the heavy instead of mines, or giving the sniper the option to switch to dual pistols as a secondary instead of two nades.
Then off course offering different variations of nades to equip with such as stun, detonation on impact but with a reduced damage radius, and the standard nades used currently in BFII.
Nothing should alter the units abilities overall though such as armor, and speed.
Acta Non Verba