"Premium Air" Fee
Regular passengers get the minimum amount of air required to maintain vital signs--passengers opting to pay the "Premium Air" Fee get enough oxygenated fresh air to actually remain conscious and responsive to external stimuli throughout the flight.
A fee for normal drinks, including water.
A fee for lowering your seat back.
A fee for using the overhead storage bin.
A fee to reserve a seat next to the person of your choice.
There are already fees for Window/Aisle seats, preferred boarding, and check-in.
Southwest already charges for preferred boarding. The first 20 boarding positions cost extra.
There is already a fee for window or aisle sets on Spirit Airlines
The next fee will be for good customer service. You want them to listen, respond and be nice? That'll cost extra.
I believe they will charge a surcharge if your flight is not full. Also, a charge to call in and ask a question over the phone. A charge to ring your flight attendant call button, and a charge if you wish to make a seat assignment more than 24 hours in advance.
I expect to see a fee for advance booking of seats online.
I believe the next fee gold mine for the airlines is access to their airborne defecation chambers. Pay to poop.