I'd Love the ability to build on floating islands, and also to attach floating islands with ropes.
forts on islands can shoot at higher ships... and look cool.
ropes can be a great way to use islands as cover for our defensive buildings, ropes will break if the enemy ship pushes the island too often.
It would be great to make bridges between islands, so if a structure is crew-less, some crew from another one could go help.
There's a mod that allows you to use floating island's blocks to build a floating fortress, we should have the ability to build on floating islands, they are pretty useless right now.