What time of year are you traveling? Australia has some nice beaches but of course are seasonal. Especially if you were to start in Japan, you could stop-off somewhere nice in Australia.

I love Hawaii but haven't stopped there on an RTW because you can't backtrack to the mainland U.S., and the flights from Hawaii to Australia aren't terrific.

You might consider the Caribbean. I'm personally very fond of Curaçao, which has some terrific beaches and the weather is great all year. With your six flights in North America you could visit there for a few days.