Ryanair
Ryanair would not pose a major threat if they filled in the gaps - but that would involve a degree of cooperation with the other airlines, something for which they are not famous.
It would be lovely to have Milan and Oslo back, I had flown on both and the Italian flight seemed to be particularly well subscribed. Additionally a few more flights to the German and Polish speaking world which I feel is massively underserved at NCL would be nice. Ryanair does not have to be based here, just fly here from some of the bases they have elsewhere.