Welcome to the Guild, Ex Machina!
You should introduce yourself in the Intro Forum.
How to make the world end, interesting question, and I'm surprised no one has ever put that before us, hmmm. I'm content with Tolkiens' method (standard method) to map only those parts of the world that pertain to the storyline.
For instance mapping the Earth for a European based story, no need to anything except Europe - if that is the confines of your story.
Waterfalls at the edge of the world? The River Police would raid you (oh wait I'm a member of the River Police... who seek geologic normality in maps expecially regarding rivers and oceans.) Actually that sounds like a viable idea, if that fits your storyline. I like it. Making waterfalls is a challenge, but its been done.
Celtic island regional - a commission
Here's a link to a map where I placed a waterfall in the ocean, as per a publisher's request - I think it worked well enough, might give you some ideas for waterfalls at the edge of the world.
I suppose doing an entire flat map of your entire world, or cut to fit on a globe, or even placed on a globe are viable options. It really depends on what your goals.
Regarding creating the whole world for a novel that only focuses on one area, I thinking showing the map of the whole world, might confuse the reader. Would it not be better to just map what's in the novel - ie: Tolkien style, as you call it?