A few suggestions that could be added to the mix:
Away from the aircraft:
1) The terminal built by a particular airline should be able to allow interline with other carriers using different terminals. This is what happens in real life with airside terminal connections and transfers.
2) Customer loyalty programme. The more customers you carry, the more your scheme costs you. But the cost is actually an investment and up the individual airline as to the reward of the scheme..... upgrades, free flights, discounted flights.
3) Limousine service like Virgin and EK provide.
On the aircraft:
1) Boarding drinks
2) Onboard wifi
3) At seat power
4) Standard TV screens as opposed to HD screens, or here is a thought, 3D tv screens
5) Duty free cart
6) Bar configure option like the EK A380 has
7) Washbags - different options for economy business and first
8) Pajamas for first class and business class
9) Slippers for first class
10) In flight entertainment paid, inclusive, extra movies, extra games, extra music options