Again people are putting that spotter hats on before their business one.
TK would be good for near East destinations but nowhere that's not already covered by BA or AFKLM save for a few more rustic destinations within Turkey. IST will be just another hub flight (of which NCL has enough of) and will compete with EK and jeopardise the rumoured second daily DXB.
MAD is covered with IAG at LHR. Vueling hub at BCN so will push EZ off their longstanding route and will connect pax to flights already connected via LHR or CDG/AMS.
LIS possibly. I believe EZ has a base there. But if TAP were to operate it then it's yet another hub. Yes there could be a couple of interesting South American destinations not served through IAG but the amount of pax from NCL to these places must be fairly tiny.
All of these proposals will not stimulate more passengers but will just give them another possible routing to get to their destination. This potential increase to eight hub flights is not sustainable as yield would suffer.