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.