That already exists: GBAS is the aviation equivalent to RTK. It provides integrity monitoring and differential signals via VHF. It's better than WAAS, since it's ground-based, and accordingly significantly harder to drown out in a large area.
But neither WAAS nor GBAS are authenticated, so are ultimately both are spoofable.
But neither WAAS nor GBAS are authenticated, so are ultimately both are spoofable.