I expect it will be like a build-your-own magazine. Maybe you subscribe for 10-15 dollars a month and get to configure a list of 5-10 substackers who would take an equal share.
It's already happening. It's called Every, it's a writer collective that works exactly as you describe. Its founder was the first Substack employee, that resigned to become a writer full time.
Every is a business oriented collective, just to clarify
Or better yet, add a carnival feature where you pay x amount and you can create your own Substack reader, like building your own personal magazine
Theoretically, they can already do this with Substack Reader, though the feature isn't available yet. But it wouldn't surprise me if they had it coded up and ready to go already.