The most profitable frequency (AFAIK) is 1, you can run more frequently on a route but it doesn't really make you any more money, it just adds to the fuel cost.
And honestly most of the long haul aircraft out there are twinjets now, the only case where 747s are being used massively are in cargo airlines. If cargo does get implemented into the game then I would argue more for fixing the maintenance model, because then larger aircraft would be much better for an airline.
I will say though that it'd be nice to include some sort of bonus for fleet commonality. Some sort of percentage reduction in the overall maintenance cost of your fleet, provided that a certain percentage of your fleet is one aircraft, and that you have a large number of that aircraft in service.
In all honesty, the game requires a complete overhaul. Yes hotswap is one of the major things, but the thing that hasn't been modified since iirc 2008-9 is the pricing and maintenance models. List of things that need a overhaul:
- Pricing (aircraft, profit, costs etc)
- Maintenance
- Statistics (There is a whole bunch of stats that can be implemented alongside the other additions)
- Scenarios
- Plus some other things i cant think of at 2 in the morning