UA and AA offer OW award travel at half the RT mileage, and AA offers OW between Europe and the US at 20k (instead of the usual 30k) offseason. You may find it easier to accumulate miles on two airlines (perhaps using credit card offers) than on one. UA's partners include LH and LX. AA's partners include BA, AB and IB, though award travel on BA should be avoided due to high surcharges.