For such a long trip I think the best would be to have a "backbone" of the trip with business- or first-class RTW ticket in combination with other flights, using various Visit XXX Passes and other options available.

As the number of flights on the RTW ticket is limited to 16 and First class is not available that widely, especially on short-haul flights, the RTW ticket alone may not give you all the destinations you want to visit.

For example in our DONE5 we will extend the trip in South America with LAN's Visit South America Pass and in Asia our DONE5 will take us to Singapore, wherefrom we'll take other flights for Vietnam, Kambodia and Thailand.