I think the Firefly Bush is a fantastic addition to the game. Having ambient particles floating around really adds some atmosphere to builds. However, I think it'd be cool of Mojang added more of these to the game. Here are just a few of my ideas:
Butterfly Bush: Found exclusively in Flower Fields, emits animated butterfly particles
Rafflesia: Rarely found in Jungles, emits fly particles. Zombies are also attracted to this when they're not attacking something
Spitfire: Dug up by Sniffers in the Nether as a shrub that has to be grown. Emits smoke particles. If it's planted on a Magma Block, it emits small fire particles too
Excelsiorchid: Uncommonly dug up by Sniffers in warm biomes. Does nothing on it's own. When an entity touches it, it sprays a fountain of firework particles into the air, then goes on cooldown for a few seconds. Can be "fed" dyes to change the particle color. Feeding it multiple dyes adds more colors
Dust Diamond: Found on the ceilings of the new ice caves. Can only be placed on the undersides of blocks. The center of this flower glows in the dark. Emits snow particles
Blowball: Has a small chance to replace Dandelions in cold biomes. The colder the biome, the higher the chance. Passively emits puff particles. When an entity touches it, it emits more puff particles