Coral blocks are one of the most gorgeous blocks in the game. Since coral is a living organism, it stands to reason that it should be able to spread. In real life coral grows on hard structures underwater, most commonly rocks. I propose that, much like real life, coral or coral fans transform stone blocks they are attached to into a coral block of the corresponding colour.
Like saplings, a coral fan or coral may go through a number of growth stages. Coral and coral fans should use their random block tick to check their attached block is stone or cobblestone and, if it is, they should advance to the next growth stage. Once the coral has advanced past the final growth stage they should spread to the attached stone block, transforming it into a coral block of the corresponding colour. This process can be hurried along by bonemeal.
To prevent coral from transforming players' builds, they could use halfslabs or double halfslabs. Transforming coral could happen naturally or semi-automatically using a complex redstone machine.
Help us restore our precious coral reefs in real life and in Minecraft.