Here are some suggestions
Airlines can own airports (majority shareholder has control)
Slower game speed 5 min per day (enough so new players can see a change trust me. Aswell as this it is a manageable speed not to quick and not too slow.)
Game changed so it says for example 1st November 2018 instead of week XXXX
Airlines can buy shares in other airlines (majority shareholder has control if equal partners then player who created retains control unless those players trust each other enough to have shared control)
Airlines sign fuel contracts and acquire fuel at lower prices
Airlines can own other things within the aviation industry e.g. Aircraft leasing company,aircraft maintenance,catering,logistic companies like DHL,dnata etc.
Cargo airlines
Better UI ( will send concept designs)
Player owned airports decide rates for shop and restraunts
Airlines can buy slots and negotiate prices (based on supply and demand
Airlines can own hotels
Airlines can own travel agent shops
Staff control wages,recruitment drives,firing and redundancys
Airlines can build training centres
Frequent flyer program
Instead of new aircraft market you get an order screen
When placing big orders with aircraft manufacturers airlines get bulk discounts
If they have placed big orders with that aircraft manufacturer before then they get a discount on their next order no matter the size (like Ryanair only operates Boeing aircraft so they get a big discount or although lion air have some Airbus aircraft on order the majority of their fleet is Boeing therefore they are eligible for a discount)
Airlines can sell package holidays
Airlines can own aircraft manufacturers
Airlines can lease aircraft to other airlines or sell
When airlines are declared bankrupt they go on a market and other airlines can buy them out from administration.
Player owned airports have to be micromanaged (buying equipment like baggage buggies ramps to load luggage and cargo,catering trucks,fuel truck, fuel contracts,catering centre,terminal building management like adding gates adding more space for commercial premises,staff management,air stairs,maintenance etc.