Would you consider including a photo safari in your plans? These can be amazing trips where you'll hang out with lions and leopards and giraffes in their own houses, not in zoos. Johannesburg is the stepping off point for some of the best safaris, and I'm told that Cape Town is one of the most beautiful cities in the world. (I'll have 1st-hand info later this year.)

If that seems interesting, it's fairly simple to make a 4-continent trip instead of 3.

(And if you haven't noticed, planning a RTW is one of the most seductive things you can do with the intention of keeping your clothes on. As soon as you're certain of what your plans should be, some even better pops into your head and you're back somewhere between Square 1 and The Wing.)