Is your feature request related to a problem? Please describe.
The existing shells cover block-breaking and shrapnel effects, but there's a couple of other roles that cannon shots can reasonably fall into.

Describe the solution you'd like
There's a couple additional shell/payload types that I think would be interesting:

1. Firework Shell. The older brother of the firework rocket, it does relatively low damage, and doesn't break blocks on impact--though when it goes off, it might set something on fire. Its main use would be in absurdly large fireworks displays, or as a 'training' shell for easily identifying where a shot would land.
2. Tracer Powder. An alternative to the powder charge, it provides about 50% of the velocity of the powder charge, but makes the trail much brighter, and makes it linger for a while longer. Best used by adding it to the end of a stack of normal powder charges. This could also come with making the initial trail on regular shots fade or scatter faster, which would make it a bit harder to gauge where a given shot landed--but if you use tracer powder or a firework shell for spotting, that helps.
3. Potion Shell. Crafted from a large supply of potion, this shell disperses its payload on detonation by scattering lingering potions. Violate the Geneva Conventions! Or possibly provide a different kind of fire support with fire resist and regen shells. Just don't mix up the regeneration shell with the harming shell...
4. Chain/Rod Shot. A mid-ground between Solid Shot and Grapeshot, this breaks blocks like a Solid Shot does, and in a wider area during flight, at the cost of a smaller impact crater. Still really dangerous for anybody who happens to be standing in the way. It should also be marginally cheaper than Solid Shot, since you're stuffing two smaller balls and a chain or rod into the area that a single larger shot would take up.
5. Kelp Shell. An enormous beanbag round, made from a slime block surrounded by kelp. Doesn't technically do damage! Does, however, knock whatever it hits into the next biome. It bounces, too, though it loses some of its oomph once it's bounced.

Describe alternatives you've considered
N/A

Additional context
N/A

bump, but potion shell somewhat implemented as fluid shell.