Personally, I think cannons should have no strength. Zip. Nada. Zilch. Zero.
And the silly bombard function should be removed.
They should have only one function, ranged attack, which would reduce city defences AND Cause collateral damage with each shot.
I think it should be made powerful enough to be effective in the field or cities, with maybe just a moderate bonus for cities like +50%
I'd also like to see some internal distinction between small arms ranged attacks, and siege ranged attacks. So that I could make Monstrous Creatures more vulnerable to only the latter.
I'd like to see siege weapons work like naval support in CivRev, by boosting the attack strength of a unit attacking an adjacent tile.