With current storage technology, a very small tag could contain an enormous amount of data and transmit it rapidly. Megabytes or even gigabytes would easily be feasible, even though we normally don't use RFID for that much data.
Used on products, we could include all sorts of information about the product, things that wouldn't fit on the label, including safety records, labor standards, environmental impact, and information about the supply chain. This would dramatically increase transparency and make it much easier for consumers who care about being socially responsible to make informed decisions. Businesses have little incentive to provide this information on their own, but they could be required to do so by regulation or incentivized by tax and subsidy systems.
In the opposite direction, RFID tags could provide businesses with even more detailed information about consumer purchasing patterns, enabling businesses to use computer algorithms to project future purchasing and target advertising. Because businesses have a powerful incentive to do this, this process is already underway. It likely increases economic efficiency, but it also raises serious privacy concerns, because a person's purchasing habits can tell a great deal about them, and could reveal potentially embarrassing or damaging information.
RFID implants are already used to great effect in animals: pets can be implanted with RFID chips to ensure they can always be identified in case they are lost. This dramatically improves the odds of a lost pet finding its way home.
As for humans, I think there will be a lot more resistance on ethical grounds, but if security and privacy concerns are properly addressed it could catch on. Instead of carrying a wallet full of cash and cards, you might only need an RFID implant to verify your identity and make purchases directly from your chosen accounts. The main concern I have is security; currently, RFID has a very poor track record of encryption (I've linked a famous case where fare cards in the Netherlands were hacked), and unless this improves people should not trust such implants. If encryption improved, this could actually be more secure than cash and cards, as an implant is impossible to steal without physically injuring the person who is implanted.