My idea for snails, slugs, and mud comes from a love of snails.
The snail and slug would be small and peaceful mobs that would overall just make minecraft a tiny bit more realistic. Having a certian percentage to drop a slime ball, they can be found in caves, plains, jungles, the ocean, and forests. Along with that, they are more likely to appear during and directly after rain!
My other idea would be mud. I believe that it should be a slippery substance that appears during and after rain on some dirt and grass blocks, maybe just puddle slightly in areas? Pigs would have a love for it, often found playing in and slathering themselfes with it. Snails and slugs also have a possibility of spawning near mud.