This certainly is an interesting discussion point. Many of the routes could be viable. However, given the consolidation that has happened in the industry in Europe and the US (Europe not done yet), airlines are cherry picking what they believe to be the most profitable routes to deploy their aircraft. Shareholder demand plays a large part and a maximisation of yield and optimisation of the schedules are to the fore... Nonetheless I'll add my two pence worth.
Coming at this from a Northern Ireland then an Ireland perspective ;
Belfast International - Toronto ( was traditionally a well established route, Canada has large NI diaspora) surprised Norwegian or Westjet haven't went in though, I'd imagine Westjet looked before..
Belfast City - Cork: bring this route back pure financial crash and air crash by Manx 2 Aer Arann had very respectable business schedule that was well subscribed. The drive is long, and I believe this route is probably the most viable routes inside the Island.
Belfast ( either airport) - Naples, Venice. Huge demand on seasonal basis. Peak summer flights on holiday routes tend to be packed and fares very high.
Time is also right to bring back Belfast to Gdansk and Warsaw in Poland. Large Polish continent in Belfast, all of who fly via Dublin.
Belfast (either airport) - Berlin. Was withdrawn in the worst of times. It used to do well and Berlin is great city break.
Belfast (either airport but probably at City) - Madrid, by Iberia express. Willie Walsh stated that despite the failure of Vueling on BCN, they'd look at opportunities ex Belfast. He said if route development fund was offered, Madrid by Iberia Express could be a real possibility.
Dublin - China and Thailand. But the former is reported to be coming on line from winter next.