I do think that MAG will want them at Stansted, but I think the sheer volume of traffic Wizz have now built up in Luton is going to make them very reluctant to consider moving. Surely there would have to be a heck of a good deal in order to get Wizz out of Luton.
Also, Stansted is fairly full up until around 8 or 9 am, so the early morning period when about half the days flights are would have difficulty and Gatwick is full at virtually any time and Southend is not really viable for reasons LTNman says.