Heat is no longer a big problem with cannons, although not entirely solved.
Expense IS a problem and is what we are trying to fix this turn with standardized bores and machine rifling.
accuracy, I would say it is not a problem. What IS a problem is our lack of area of effect shells.
I agree. The enemy has accuracy, and area of effect ammunition. We don't have either. We have pretty good, yet still mundane, accuracy and low impact area. A very similar arms race actually happened during the American/Soviet Cold War. I don't remember the exact specifics, but it came down to the Americans having more accurate missiles or IBMs or whathaveyou. So what did the Soviets do? They just built bigger bombs. You don't have to be as accurate if your explosion range is large.
If we do anything else with our cannons in the near future, it should be some sort of explosive ammunition. I'm partial to
Tower of Frost Ammo Permafrost Ammo. Basically we just take the principle of the minor frost towers and do it again. Instead of one big one, now we have several to increase range and power. If we put the frost spell into our ammunition, and we pelted them with micro-frost towers, then they're going to get cold really fast.
Works by turning the inside of a hollow shell into a frost tower. The spell builds up inside of the shell during storage. When the shell lands and is damaged, it releases the built up frost spell in a frost nova. Then if its not absolutely shredded, the shell continues to give off the Forever Frost spell.