I've read the ESA wants to have an astronaut spend a long time aboard the ISS. It's discussed with the Russians.

Cosmonaut Polyakov singlehandedly convinced the Russians it was a good idea to have him spend 18 months in space aboard Mir. He had heard in 1992 the americans wanted to have a crew spend a whole year aboard Mir, so he took a further step, talked about it to his boss and got a trip to the station, as it would be good for Russia to keep the endurance record. But it only lasted 14 months, as Norman Thagard was going to occupy the station and Valery had to come back (the station could only allow 3 spacemen to stay aboard at all time, except during visits).

Raoul