Right now, all route demand is in relation to the proportional size of departing and arriving airports. For an example:
LHR-ATL would have high demand, similar to the real world.
LHR-MKE would have 200-400 daily pax demand.
LHR-MSN would have like 50-100 daily pax demand.
in reality:
LHR-ATL would perhaps have a boosted demand due to connections
LHR-MKE would have ~50 daily pax, not worth a once weekly flight
LHR-MSN would have no demand
What's happening here is that connections are more prominent, meaning that Passengers wanting to go LHR-MSN for an example, would simply connect through ATL, meaning more demand (or at least more blue pax). Then however, regional flights from Large-Small (and vice versa) would have more pax. Regional flights from Small to Small would have no pax.
Here is my suggestion:
Large-Large: more pax
Large-Medium: less pax (but not none)
Large-Small: No pax
Medium-Medium: Slightly less (more if there is a hub)
Medium-Small: Less pax (but not none)
Small-Small: No pax
Regional Routes would gain demand,
Long haul routes would gain demand (only between larger airports)
All other routes would get less/no demand.
All in all, this would obviously control airlines from crazy senseless expansion, and would prompt airlines to be more realistic. To add to realism:
-All services/qualities of your airline would have much larger influence on popularity and prices.
-Operating costs should be cranked up (maintenance, fuel, management...)
-Terminals should be more controlled
-Aircraft get delivered in a backlog
-Blue pax to be more from larger airports (less from smaller ones)
-Shared Gates
I'm well aware that this has close to no influence on the future of this game, but I'm just adding my opinions to the pool.