This has been mentioned before, but I think it deserves a bump because of all the things that would add a new dimension to escalation gameplay, I believe this is one.
What makes adding artillery a big deal is that it gives ground units a much more effective way to threaten airbases, factories, and other structures that players might care about.
Right now while ground units aren't inconsequential, the threat they pose to strategic assets is very... slow. Ground units are only ever an eventual problem and never anything you actually worry about.
Having groups of units that can attack strategic buildings from afar would make them much more meaningful to attack or defend.