I would love a FRA connection, DUS is very much akin to having a link to STN (nice to have but not as good as LHR). A flight in the middle of the day if you are connecting on to Europe or North America is not ideal either.
Recently I had to go to to LNZ in Austria which is basically a Star Alliance/bucket & spade airport and ended up getting a train to EDI to get a flight to FRA and then on to LNZ. NCL-DUS was a non starter as the later departure time, 2 connections plus the horrible non Schengen>Schengen terminal change at FRA would have turned a half day meeting into a 3 day trip.
Star have such domination in Germany, Austria, Belgium & Switzerland and a FRA connection would give us better access to secondary cities in these countries.
However from the perspective of FRA and LH, they probably do not see a NCL link as high up on their priority list. Maybe a double daily (morning/evening) DUS would be the most realistic improvement we can hope for?