What are NCL's 8 hub destinations?, I count:
LHR, AMS, CDG, DXB as the ones people look for and consider real hubs with many varying choices of destination. DUB only offers transatlantic connections, EWR will be coming online which with Dublin make 4 hubs and two technical hubs, although the wide variety of US destinations will be welcomed. BRU? I think connections are offered but how reliably, same applies for CPH and DUS, I wouldn't really be counting those three, I think they are mostly O&D.
Istanbul would definitely be a good addition, it might steal *some* passengers of EK, but I think both would see growth, TK would likely become the most reliable option to TLV which would serve the large Israeli population, along with near east, much smaller cities than EK (I don't know how TK make them all work!) and again, more options to Asia and possibly a more sensical route to Africa. Plus Istanbul is a big tourist destination in it's own right
Lisbon, again, a tourist destination in it's own right, very nice coastal towns etc for 30+ miles around it and as mentioned, South American and African connections along with destinations like Madeira, they can be cheaper than TOM/LS and combine the two destinations.
ZRH/FRA, I know what you mean, not as much O&D, very similar markets, but until UA, NCL had no major Star Alliance carriers... Bigger offering might be nice (but not necessarily profitable)
MAD, controversial but IAG swapping one NCL-LHR for one NCL-MAD? Connecting to South America and Africa + European destinations?
Vueling, cheap connections, might result in EZY reducing frequency but I think they would hold their own.