A Stansted base isn't such a crazy idea. Jet2 will need to expand into the SE one day. Surely.
Other airports cope quite well having two or more carriers on similar routes. There's no reason why the UK's fourth busiest airport can't diversify and attract some competition from carriers. Don't Jet2 compete directly on routes from EMA, MAN, EDI and probably other airports with FR? Jet2 also offer package holidays and are now quite big on this. I'd see their main competition as Thomas Cook or Thomson at STN to be honest.
I actually think a Gatwick base would be a worse decision than a STN one if they want to set up shop in the SE. I'm sure the time will come, sooner than later?