Canister shot has been a feature of Nations since the very beginning of N1 so I'm not making this suggestion lightly. However, after going to multiple sieges in n2, it's become clear to me that canister shot has outstayed its welcome.
On N1, canister shot could be devastating if you coordinated lots of cannons to focus on one player. As far as I can remember nobody complained about that back then. However fast forward to n2 and things have changed. The meta has developed so people can and will spam cannons from forts that are much harder to fight than n1 towns and on top of that, peaceful town abuse makes this even more difficut to counter. The people and their mindset also much more sweaty now, so they will abuse this as much as they can and do anything to win even if they know it's not fun. As a result canister shot is giving a huge advantage to defenders.
However this is not because canister shot does a lot of damage per se, it's because it can be mass produced, spammed and focused on the banner whenever the attackers get the upper hand. This and the fps lag it makes disrupts pvp and makes holding the banner as attackers really difficult. All this means canister shot is more annoying than something that's interesting to use and plan counter measures around. If you go to a siege you see a lot of people complaining about cannons, not because of the explosive ammo, but because of canister shot, and even cannoneers will admit that they use canister shot to annoy and stop pvp than kill.
Some people are going to say make canister shot more expensive and more damaging, but that won't solve the fundamental problems with it that I described and actually it will just amplify them by making them into a kill farming tool like they were pre-nerf at Germania. Others are going to say that it's the attackers fault and they should use better tactics by destroying defending cannons before capping, and I agree with this, but even if attackers always do everything flawlessly, and they won't, the defenders can rebuild their walls and cannons very, very quickly
In summary canister shots are hard to balance, they will either be OP for shooting people at banners like they were before, or useless but very aggravating like they are now. In both cases they worsen the siege experience and their removal won't be a loss to the server