The rewards segregation based on community size (or better yet its cumulative SP) would make the rewards fair, while maintaining the difficulty vs reward ratio equalized across the platform.
Unfortunately, it would also cap the max a post can earn to the communities rewards power.
Perhaps I am thinking too ahead of the time, I see this more applicable to when Steemit has millions of members.
So in the current stage, I think your solution with dolphin curators voting on steemit's SP might be a lot more practical.
I agree that ultimately something more like your idea is good. but one step at a time, minimal changes to the existing system that solves the current obstacle. Iterate and we dominate. Stagnate and ...