I've been trying to find a tool that could give me a list or map of all the destinations of a certain airline.
Something like this easyjet's route map, but for all airlines.
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The interface is a bit inconvenient and it has a limit on the number of routes shown simultaneously, but you can do it on Kayak's list of airlines. Click on an airline and you'll get a map of where it flies. E.g. a map for Easyjet:
You can then filter by a specific city to get a map of which routes are available there, e.g. for Easyjet in Prague:
In addition it's possible to show all direct flights from a given city, but without an airline filter through their Routes page:
Between the Star Alliance route map, the oneworld route map, and the SkyTeam route map, you can see the vast majority of existing flights. For any given airline, just choose the map corresponding to its alliance. And since most itineraries will have all segments within the same alliance, you probably will need to use only one of these maps.
You could try openflights. I'm not sure if the data is complete, but a map for easyjet:
And Indigo (India LCC):
I have found Wikipedia a particularly useful resource in this. They usually have a page for say Easyjet destinations and also the pages for airports list where airlines fly from there. It being Wikipedia is not updated automatically but then again, it's Wikipedia so it's reliable... enough.