Evaporative cooling.

As long as the new biggest integration opponent is less opposing to integration than UK was, it will still be beneficial to integration efforts.

I propose NL under Geert Wilders. It will be a total disaster but it could actually happen.

Germany has benefited from being able to be a swing voter in the council votes due to QMV. With the UK gone it will now have to nail its colours to the mast if it wishes to resist significantly more redistributive and socialist policies supported by the southern countries, instead of acting as a decider knowing what the UK will do. This could change the politics in Germany in unpredictable ways.

Money transfers are necessary for eurozone to stop blowing up in people's faces, sadly