Simply Default datapack helping

So I decided to stop looking for Other People to make My datapack and just to make it Myself. I've been wanting to add a few new items such as pufferfish strips You can get by putting a pufferfish in a stonecutter, a zombie arm that's rarely obtained by killing a zombie and lets You have the same attack stats as a zombie, and when it's daytime You can cause fire damage. Mobs, which are Ogres found in forests, stronger and slower than zombies, Small Gremlins, which are magma cubes spawned in Strongholds, Bunnies which hop around doing melee damage and explode on death. New biomes, mainly themed around removed and prerelease biomes. Mob spawning changes, erasing creepers from existence and making skeletons spawn more rarely and not burn in sunlight, and much much more, but I'd want to start with what's easier to figure out.