What will be the next airline fee?

I believe the next fee gold mine for the airlines is access to their airborne defecation chambers. Pay to poop.

I expect to see a fee for advance booking of seats online.

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.

The next fee will be for good customer service. You want them to listen, respond and be nice? That'll cost extra.

There is already a fee for window or aisle sets on Spirit Airlines

Southwest already charges for preferred boarding. The first 20 boarding positions cost extra.

There are already fees for Window/Aisle seats, preferred boarding, and check-in.

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.

"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.