After playing a (rather short) round with too many islands and reefs, I thought about the possibility of transferring cannons from a stranded ship to the island it is stuck on (not including reefs, only land with enough "flat" area for a small battery).

You are able to decide if you want to bring one or both broadsidebatteries ashore, the damage they are able to inflict depends on the current state of the ship (full damage for all guns, halfed damage (rounding up) for the guns of one broadside). Of course such a land battery wouldn't have as much hitpoints as a fort, maybe the damage it can hold should depend on the status of the ship (or be only one field with the burden of the ship, symbolising the number of single cannons that can be destroyed by hits).
My first prototypes are made of sawn off, black-painted 1mm nails as cannons, glued into a block of five wooden gun carriages with a vertical "flagpole" (another nail) with which it can be stuck onto the island (I'm using custom made islands and reefs, mostly from painted wood or polystyrene) and be turned (It is not a real fortification, so it should be able to change it's position rather easy). In this case the firing arc should be limited (maybe no "real" arc, but only the combat ruler, placed in front of the battery line, should determine the range.)

This is just a raw idea, not yet ready for a final ingame-version (and in no case a flawless concept!), so I hope for ideas from your side.