Even though the caliber of all cannons tend to go up(basic cannon> heavy cannon> imperial cannon), I feel that it may be a bit lacking in variety. After all, you can only go so far until you run out of way to "shoot directly at something in front of you". Of course, you could always implement grapeshot and various shotguns ( hint hint ), but that still fall in the same category. Which is why I would love if mortars, howitzers and others catapults were implemented in the game.
Naturally, such devices would be fairly heavy, slow to aim, have a long travel time and generally be expensive, but that would be offset by the massive amount of damage these shells/cannonballs would be able to cause both by their sheer size and/or payload.
As in real life, that would restrict them to being put in building or VERY heavy landships that would most likely be dedicated to crushing buildings or the aforementioned ships. You certainly wouldn't find them on light aircraft.
Now, I don't know how the physics of the game works, but I don't know if it could be possible to have the mortar crew restricted to land targets.
And since the shots travel very slowly, could moving targets perhaps avoid the projectile entirely?
So there you go, my ideas.