Personally, what I want from siege weapons is to revise how they work. Look, in my philosophy of civ game, units warring is a separate thing from city/empire development, OK?
So what I want is this:
Siege weapons in the above-mentioned line don't have ability to attack units like usual. They can bombard but is much more random to simulate enemy's skill at dodging stuff. No promotion here, the actual random skills. For city attack, when bombarding fortification, they can NOT do collateral damage until the defense is at most 5% or so and evne then, the chance for successful CD is fractionally smaller than if there was no city involved.
That scenario would balance my usage of siege weapons because I usually have the stack of
5 swordsmen line units
5 archer line units
5 horse military line units
5 siege line units
Then
5 gunpowder infantry-style line units
5 gun-toting horse units
5 artillery
Then
5 infantry
5 tanks
5 helicopters
5 artillery
So, needless to say, these stacks roll so easily over cities with how siege weapons are designed now. If siege weapons' CD is revised, then 1) siege wapons are still useful for bombarding fortifications and 2) lesser CD and more experiences for my other units in the stacks

.