As stated above Blue1 (SAS owned) and Finnair allready operates on HEL
so I guess it would be rather hard competition.
Norwegian operates using only B737-300, and that might turn out to be to large for such a service. Not many leisure travelleres between the two capitals I would guess.
Most likely Norwegian will start more international routes from Bergen, Trondheim, Stavanger and Harstad/Narvik.
Norwegian allready announced their start up on Hamburg - Bergen.