Im curious - where does AS get its route stats from?
For example - if I were to connect 2 small towns together, but in real life no such route exists, would this flight fill with passengers?
Or does the passenger demand data come from real life routes only? If so - how updated is this information?
Top secret...
there might be a demand. sources are secret, but you do not have to fly routes that exist in real life only. however, when it comes to very small airports, direct routes usually don't really yield many pax. if they fill, it is usually with transfer pax.
Trial and error is king I'm afraid
In general, domestic routes and shorter routes seem to be easier to fill than longer ones or International ones. But as Ianmanson said, you'll only find out if you try.
However, you can orientate by the real traffic demand. Just google your airport and take a look at the airports traffic statistics
As somebody corrected me: this Methode only works for big airports 
This won’t really work for small airports. Trial and error plus connections will make it.
Top secret? LOL
Awesome - thanks for the info!