Like a today very popular topic "new airlines best base", if you will to start a low cost in Europe, where do you plane to base your hub??
Stockholm Skavska (close to Stockholm high fares traffic)
Hanh (close to Frankfurt)
Milan MXP (it's a big airport but with a lot of slots available)
Berlin (the new Brandebourg when it will open)
and finally Lyon (france) cause it's in the middle of Europe.