I love bicycle touring, and I look forward to another 7-10 day tour this summer. I have no doubt that in the future I'll do a full cross-country tour for 3-4 months, hopefully with my wife.
Even with that in mind, I feel like planning for more. Long distance hiking is like cycle touring, but on 4-7 day backpacking trips I've become a bit bored unless the scenery is fantastic.
Bumming around Europe would be fun, but I'd do it by bike.
For something global, I wonder if cruising by sailboat wouldn't be interesting. I really like the idea of taking 3-5 years and doing a comfortable circumnavigation in a 32-foot bluewater sailboat. At first, it doesn't sound much like cycling touring, but now I'm not so sure. There is a lot of work to be done on a sailboat at sea, and the lifestyle can be very healthy. The idea of making landfalls after a long passage sounds great, and then exploring islands, ports and other countries sounds really exciting.
Sailing away from it all is a common dream-- one that isn't so different from cycling away from it all. What other dreams (or experiences) share similarities with cycle touring?