Without XL and their associated tour operators, NCL will loose destinations such as EFL, JSI, KVA from their route network and frequency to TFS, PFO, CFU, FNC, ZTH and DLM will decrease. Kosmar, part of the XL group who collapsed along with the airline were the UK's number one operator to Greece and operated to many smaller islands in Greece from smaller UK regional airports. Considering the current state of the industry I think it is highly unlikely that many destinations will be reinstated by alternative operators. having said that, most XL flights were very popular from NCL - particularly the Greek islands. NCl cannot afford to loose many more destinations or expansion will be affected greatly.
XL will be a miss in the industry and good luck to all the staff affected.