6G will be much faster than 5G and will use submillimeter waves. On one hand, the higher frequency will support broader bandwidth. On the other hand, because of atmospheric absorption, the range will be shortened and hence in order to have adequate coverage the density of cells will have to be increased significantly. The cost increase might become too high to bear as a business.
Can we find resources of lower cost as an alternative to building a lot of towers? We have to think outside of the box. Let us look at Uber's business model. Uber uses the extra capacity of private car owners to transport general passengers. Why can’t 6G uses extra capacity of individual cell phones as transponders to extend the range of cell phone towers? To do so, link margin and cyber security are the two key issues from a technical point of view. If a cell phone has enough link margin to communicate with a cell tower 200 meters away, it should have enough link margin to connect to another cell phone 200 meters away while the other phone can be almost 400 meters from the tower. That is, by using that cell phone as a transponder, the range of the cell tower can be extended from 200 meters to 400 meters. If two hops are allowed then it can be extended to 600 meters. On the security aspect, the current cyber security technology should have adequate separation between the channel used by the intermediary cell phone owner and the channel used by the transponder so neither side will leak to the other.
As a business, compensation is the key since no one is willing to let someone else's signal pass through his phone unless being paid. But the world trend is on sharing – from Uber to block chain. People are accepting the idea of sharing. There are many business models to follow. The other technicalities are easier to solve such as how to set up route automatically and how to transit from one intermediary phone to another when relative geometry changes. Those are mature technologies.