I could easily see Ryanair doing Frankfurt (Hahn/Main), Oslo (Gardermoen would be nice), Athens and maybe Hamburg.
I could see EZY (if willing to expand) doing Hamburg, Toulouse, Zurich.
Then EuroWings could do Munich and Stuttgart.
Istanbul unlikely but if anything, possibly Turkish Airlines.
If Ryanair don't do FRA or OSL, Lufthansa and Norwegian could pick up those.