A few things that came to mind..
*Brazil to El Salvador, what route do you plan on taking? The most common South to Central America route is through Ecuador and Colombia, but you'd be doubling up on yourselves. Is there a route through coastal Brazil/French Guiana/Suriname/Guyana? An idea is start in Brazil, come down through Uruguay to BA and Patagonia, across to Santiago and then just keep going up? You probably would have to connect through Santiago to get to Brazil depending on flights but that just seems to make a bit more logistical sense to me

Also, are there no flights from El Salvador/Guatemala to the UK? I notice you omitted Mexico entirely also..
*I agree with Louise, don't forget Malaysia! And overland from Malaysia/Singapore to Indonesia IS possible.. but you said that you didn't really figure out SE Asia yet so

*Darwin and the rest of North Australia is in the tropics, so winters will be very mild there

I never thought of that route through Aus though.. you miss everything between Perth [assuming that's your start point] and Adelaide but you fit in the red centre.. not bad
*It's possible to boat from northeast China [Beijing and nearish cities] to South Korea, overland through S Korea and then S Korea to southern Japan! Another omission I noticed, but I don't really know your tastes so I'm just working with what I can here

*I've lived in Barbados for almost 10 years and I have the most pathetic excuse for a tan ever.

It's not guaranteed! But yes you'll probably get a bit brown, haha.
I've never done a RTW so I'm also very jealous

So that's just coming out of research I've done. Hopefully it helps

Sarah: Little girl, big dreams. Currently living in Scotland
British born, Barbados raised, slowly but surely getting out there. One day..
Next trip: Barbados Dec 27 - Jan 8, Australia Mar-Apr 2010