Have your players ever said "I want to play something LARGE?" or "Can I just be TINY? A Tiny Tinkerbell?" Have they ever said, "I want to play a MERMAID?" or even "I can be whatever I want to be? Cool! I'll be a Unicorn?"
And then... you've had to let them down? That they CAN'T do that? That there's no rules in Fifth Edition to let them?
WELL! HAVE I GOT GOOD NEWS FOR YOU!
They can be a tree, they can be a fishy fella, they can be a tiny murdersprite, they can even be a full on Unicorn with a horn and hooves and everything.
And if you like this, then you'll LOVE Homebrew and Hacking: Crafting Heritages and Cultures. That's got everything you need to handcraft all those heritages that your players desire, and if you've got what you need right here? AWESOME! I'm glad I could help. :)