I wanted to recommend start-up requirements before someone could fly international routes. I may be wrong, and completely ok with that thought, but most airlines don't just start as an international service. They tend to have years of prior domestic service and then move on to begin international service. While I am in the USA, I see examples like Southwest, and JetBlue. They both are domestic airlines with international service barely starting in the last year.
My thought is a requirement be met before an airline is able to fly an international route. That requirement might be a few years of domestic service, or a set number of destinations served. I think International Travel racks up profits quickly due to demand than say a small 140y demand route that is 500 miles away.
Now I know there are airports/countries that require this to be waived due to country size and what not, Dubai, Cuba, Puerto Rico, but for the two later maybe it could be made a regional thing, like Central American. I think this would be more realistic.