Dublin would be good, aer lingus regional have set up some new routes recently and I wouldnt say that Liverpool would be impossible in the near future, aer lingus did it a few years back with A320's but pulled off because of Ryanair, I don't think Ryanair will be a problem if aer lingus regional were to do it because its a smaller aircraft to fill.
Air berlin would be great as well, I can see them coming in in the short - mid term future.
I think Thomson could do it - there is plenty of potential for them:
Alicante, Palma, Mahon etc would still be sustainable even with the other carriers, then there is Greece, Cyprus, Turkey, Morocco, Florida(!) which could sustain 2/3 weekly flights each.
Plus Jet2 will definetly be operating from Liverpool just not yet, they didnt pull out they've just delayed the announcement.