karl : if you are going for the small aircraft model, you have to be even more careful where you base ; for starters ; the USA and most of Western Europe is bad news just because labour costs are so high ; they'll eat you out of house and hangar : you'll also find marketing costs are greater too, making achieving a decent CI tougher : add in outrageous slot costs at the majors and its no wonder few manage to survive.
You'll be better off looking for a less-developed country with a reasonable number of airports you can service ; international traffic being the bonus, not the necessity. Thats purely a function of having less range to work with. So, Latin America is a place to look, as is Asia, Oceania has a few that fit the bill. PNG has prospects, 20+ domestic airports within easy range ; some international traffic if you want to expand beyond there, to Aus, NZ, and Asia. And yes, you can build a decent airline out of there, I know
Greece, and that cheaper end of Europe, has some decent prospects too.
There are bases out there, you have to look, make notes, file for future reference ( eg : I noticed my current JA base whilst playing in MT with some time to spare so looked around a bit. And maybe you find out they aint so good after all ; or they may be a little gold-mine.