You'd be surprised. I recommend a classic book called The Algorithmic Beauty of Plants. And I've seen other people do fabulous things with procedural 3D, like entire cities with a variety of architectural styles. See some related work in this thread. As for procedural textures, they are already a thing. There's at least one tool hosted right here on itch.io that generates tile art for games starting from simple rules.
As an aside, the term "procedural texture" is more often used for when you specify the optical properties of a material and the software makes it look right -- like in raytracers, though the Wavefront OBJ format also has limited support for that. But as you might imagine, that basically invites procedural generation. And painting a regular texture in memory before applying it to an object is also a valid technique I've seen recommended.