Even the World Cup did not push the DUS load factor up very much. With most of the "bucket and spade" routes for Jet2 running load factors in the high 80s (rough calculation from recent CAA data) I'd say Jet2 will be sticking with them for some time to come.
Jet2 don't see to shy away from using "busier, more expensive" hub airport (eg AMS, CDG etc). It's getting the slots that is the major obstacle. FRA makes more sense twice daily, but with an aircraft smaller than a B737-300, and Jet2 won't be going down that road. CDG would be a much more attractive prospect for more flights.
Additional flights to the likes of Rome appear logical enough but aircraft utilisation in summer is already high and extra flights would have to go on off-peak days or be at the expense of other rotations.
PTH