Anything in the corners of Europe. Iceland, North Cape, St. Petersburg on the one side, Gibraltar and anything south of the med at the other.
Up north the weather will be the most challenging, down south the bureaucracy/corruption/language.
The reality is that you can fly anywhere within Western Europe in two days maximum, provided you've got good weather. So as long as you stay within Western Europe, you really can't classify that as an expedition.
Oh, and you might want to PM Sam Rutherford. He might have a few ideas too. Read some of his posts to get an idea of what he's doing.