What I've been missing is not so much finding the best routes between two airports, but finding the best (ie. least expensive) route from a bunch of departure airports to a number of destination airports.
If I'm in Antwerp, I really don't care much if I have to depart from/return to Brussels Zaventem, Charleroi or Eindhoven, it just doesn't matter. And if the fare is sufficiently cheaper, then Amsterdam, Düsseldorf, Beauvais or Paris is fine too. And I don't have to return to the airport I left from. Typically it's the same for my destination in the case of holidays.
But I still haven't found websites which allowed to do that easily, apart from Google Flights maybe.
It's not perfect but skyscanner.net does the first part of your description. I just did some test searches from Eindhoven + nearby airports, and it found routes starting in EIN, BRU, CRL, and ANR. By default it shows different take-off/landing airports.
If I'm in Antwerp, I really don't care much if I have to depart from/return to Brussels Zaventem, Charleroi or Eindhoven, it just doesn't matter. And if the fare is sufficiently cheaper, then Amsterdam, Düsseldorf, Beauvais or Paris is fine too. And I don't have to return to the airport I left from. Typically it's the same for my destination in the case of holidays.
But I still haven't found websites which allowed to do that easily, apart from Google Flights maybe.