There are enough discussions on whether or not the cannon should be removed. But assuming you're keeping it, here are some minor UI annoyances and how I would go about solving them:
1. **The sound is annoying.** Sometimes you hear it every few seconds without break. It gets on my nerves. I'm not sure what the best solution here is, but it would probably help to attenuate the sound massively if the cannon destination is far away from your current location. I want to hear it when my lanemate or enemies use the cannon to re-enter my lane, but I don't need to hear it when the other lanes do the same.
2. **The targeting mechanism feels very clunky and awkward.** In my opinion, the left click should retain its normal behavior even while using the cannon, with only the right click resulting in the cannon being fired. Usually, what I want to do is enter the cannon, then click the camera to view the general area I'm targeting, and then click on a specific spot. **Right now, the only way to do that is to right click the cannon and then very quickly left click on the minimap** where you want to land, before your champion actually enters the cannon. That way your camera will be in the correct position, allowing you to fine-tune your landing location. This is a very clunky and awkward design. It would be best if I could just continue using my left click as normal.
3. **The cannons should trigger an "I'm on my way" ping,** rather than a regular ping. Apart from being the correct ping for the situation, that way it would let us see who's arriving before they do.
4. **The forced camera lock is very annoying**. Solution is simple, just remove it. If I want to watch myself in the air, I can just hold down the space bar. But I don't see why I would want to do this: I'm only interested in where I'm landing. Having to constantly fight the camera to re-focus it on the battle I may be entering only detracts from the game experience.
5. **The targeting range needs to be made clearer.** Similar to a Xerath or Jhin ultimate, we should see a big targeting circle. Clicking outside of this circle would result in an "out of range" error, rather than the selected position just being clipped to the maximum range. Goes well with suggestion #2.
As a final word, even if you're keeping the cannon, I think the "immobility" time after landing is a bit too long. We all make mistakes, and sometimes I end up landing closer to an enemy than intended. Right now, you become targetable before you can begin taking action. I would propose making these two the same time, i.e. you can flash away the same instant that the enemy can start attacking you. That at least gives you a fighting chance.