I think of Campbell's monomyth not as a track to follow, but as a rhythm which good stories flow through, each in their own unique way.
Instead of "What's the next step?", the question becomes "What does the story need or want right now?" the answer to which is often something from the monomyth.
E.g: The characters are daunted by the obstacles in front of them => push a potential mentor across their path, or have an existing NPC drop some helpful advice into the conversation.
E.g. The NPCs have been playing it straight for a while => might be time to throw a Trickster into the mix to stir things up.
NB: the answer may also be something much more mundane like giving richer descriptions, making sure everyone is playing the fiction not the rules, or weaving n a story thread that has been lying around.
The other thing that I find helpful sometimes is recognizing which phase of the monomyth the players are in and responding accordingly.
Can you build a game out of this - I'm not sure. The monomyth is broadbrush meta-game stuff (e.g. Hollywood uses Campbell's work to critique and improve potential scripts, rather than write them from scratch), which makes it better suited to GM tools than player mechanics.
But if you can work out how to weave the monomyth into the character end mechanics of your game in a way that's fun to play...