Actually, I'm in a similar position right now, but on a smaller scale as my funds are a bit more limited than that - thinking if I'm careful with money, I could do maybe three or four months. My plan is to tour Europe extensively. I'd like to go to all the usual places, plus Russia, the states down by the Adriatic, the length of Italy, etc. I want to visit a number of historic battlefields (Ardennes, Normandy, Peninsula War, Cassino, Stalingrad, Mortain etc). I want to ride the best of recommended 'biking' roads, ride around the Alps and Pyrenees, do the tourist bit in various European cities. Lots of research to do, and lots of things to think about for someone who hasn't done much riding outside of the UK, and that a long time ago. It may be that I'll just end up doing two or more shorter trips though.
My major problem is going to be one of fitness. I'm not sure how much stamina I have for a really long tour, and despite wanting to save a bit by doing a bit of camping, I'm not sure how well my back will cope with a lot of that now - well, yes, I am; not very well
If I can solve that little problem, it'll make planning a lot easier.