I disagree that there is any room for further lo-co expansion from NCL, the market is pretty much tied up. There realistically is only room for new routes to VLC, MAD, VCE and the new Eastern European markets. Indeed, Jet2 expansion has been tentative, whilst there is no sign of any further EZY expansion in the near future.
The only candidate would be FR as they have a bit more diversity in destinations. WW, LS, EZY and GSM are all pretty much of a muchness in terms of destinations. WW have already made DTV their northeast base, and I think GSM is a non-starter personally.
The only real potential now, in my opinion is full-service destinations - i.e. FRA, JFK/EWR, MUC, ZRH, and (possibly) ATH, HEL, OSL, ARN. Plus a bit more charter diversity - i.e. AHO, BIA, CTA, CUN, DBV, HRG, PUY, SMI, SPU, TCP, SKG.