Artillery Dice with Misfire, -4, -2, Hit, +2, +4 would be much better for cannons. It would also remove the need of chosing any other target than a model. Making it worse versus mounted monsters would be a good idea. There are some other war machines that could use a boost though, only looking at cannons is not fair to especially 8th edition empire. (The change to the artillery dice would work for the Empire Mortar as well: Misfire, 0, 0, ,0, +2, +4 making it far more accurate)
Presumably you'd then nominate a target right? Otherwise if you nominated a point on the ground it wouldn't make actual difference...
... and then I won.
The idea was to add an artillery die (or whatever other name) that has sides Misfire, -4, -2, Hit, +2, +4, not replace the Misfire, +2, +4, +6, +8, +10. The bounce would still use the old artillery die, just like numerous other stuff. It would solve the unsatisfactory targetting rules we have now, as the rules could be changed; you must pick a model as the target. This would already make the cannon a lot less accurate for sniping and characters mounted on 50mm monsters would also be safer from cannons.
Forcing all it's players to buy a new "full set" of dice does sound a lot like something GW would do.
In fairness, the scatter and artillery dice haven't changed in ... 20 years? 40K dropped the artillery dice but WFB have kept both up to now.
... and then I won.
Why not just make it near/far with a d6? No reason to add *another* special die to get lost (unless, as mentioned, you're GW). 1-3 and you land short by whatever's on the artillery dice (or 1-2 or whatever ends up working out).
The important aspect is not a new die, but the change to the targetting rules combined with a chance to fall short of the target or overshoot it, without losing the chance of a hit or misfire.
I really like Pippin UI's idea, as a 9th edition fix. Hopefully GW will actually think about what was wrong with this rule set instead of just adding another 200 pages of fluff and making us pay $20 more.