I see a lot of benefits to Ryanair service from Newcastle (on a sustainable basis)
They offer more, higher frequency, cheaper flights (ALC already up to 5 weekly and AGP starting at 4). To places like ALC, AGP and maybe PMI, FAO, TFS down the line it's definitely a good thing. It could work out as it might shake up LS/EZY and cause prices to fall. We might lose a few flights from them but that might be positive as well. i.e if EZY swapped x2 AGP flights for x2 flights to somewhere like Marrakech I'd see that as a definite positive. Ryanair also have a lot of potential to create growth, perhaps if they were to start flying somewhere like Marrakech, Warsaw, Riga etc.
However, what I would urge NCL to do, if I could, is to keep them away from areas they could cause damage, routes like Rome, Krakow, Prague, Pisa where Jet2 is established and FR could just completely blow them out of the water on. Also some holiday routes as TOM/TCX could be very vulnerable to loss of market share by FR. I could see bad things happening if FR were to start routes like Fuerteventura, Corfu, Rhodes, Kos etc. Quite big markets but ones where incumbents could easily lose out.
My wishlist for the near future off the top of my head would be Valencia, Madrid and Tenerife, maybe Chania and Lanzarote and then maybe Lisbon
I see that LS and EZY have pushed Air Malta off NCL-MLA for S16 though