The "route approval" functionality suggestion is solid. However it would need to be implemented in a simplistic form, such as it cost's $500K (yes, high...but the goal is to curb expansion by reducing enormous "war chests" of capital) to apply for route authority. After an airline applies for the route, it gets approval in like 1 month's time & pays an additional $500K @ "start-up". Also, if the route already has sufficient capacity---like an ORD-ATL in year 3 of a world---the request would be rejected & the airline would have to apply for a route that "needs" additional capacity. The effect of this might be that the airline applies for ORD-MSY & get's approval, however the profit generated would be less than ORD-ATL. Whatever can be done to curb the fantastical "war chests" is a positive.
I've started a few airlines, like Elevate in the most recently re-started "world" done well, but become bored w/ the just place an airplane on a route & even in the worst case scenario it is making money....so I have declared bankruptcy....going to "try" & stay engaged in one of the future worlds that is re-set.