I thought it could be interesting to have promotions for those units that cannot normally kill stuff, like mostly artillery and air units. you could have all units start with something like 50% maximum dmg to the enemy and subsequent unit upgrades lower that ratio. like levels 1,2 and 3 giving 65% 85% and 100% (ie kill) respectively. to avoid later, better units to go over 100%, these could already start with one or two of those promitions. like catapults and fighters start with the 1st promotion, artillery and jet fighters with 2nd and the UCAV with all three.

obviously, these have to be balanced, I'd say making them available without technological and upgrade prerequisites would be too strong, but it would be a nice touch. you could still have a light bomber that sinks enemy ships or a cannon that can conquer a city, but it would come at the price of giving up several other useful abilities from promotions. just a thought that this could be a nice game element, and feedback welcome.

Interesting, but I can see this leading to SOD's made of nothing but artillery.

a valid point, but artillery already does 85%, so it would only lack the third upgrade and this could be somewhat hard to get, like require combat 3 or several city attack upgrades or whatever. I'd say there could be a point where balance is achieved.

also don't forget that a) artillery has been lowered to 16 in LoR (though along with nerfs to other units like squad infantry and machine gun) and b) some units do flanking dmg to artillery. artillery only stacks with just these upgrades would have a hard time against armoured scouts for example, not to speak of helicopter units.