It really starts with you deciding where you want your RTW to take you. If you're not going to Africa anyway, then the marginal cost of MRU (or South Africa for DONExs) is not only excessive, but may well bump you into more continents than you would have planned in the first place.

A DONE4 starting in Japan, for example, will be around US$900 more than one starting in MRU. On the other hand, NRT can be done as a turnaround from N. America (even no hotel required if you can sleep on planes) so the all in cost (positioning ticket, ground logistics, etc.) may well be less. Same goes for Sweden ($400 more than Japan but maybe cheaper to get to than NRT.)

If elite qualifying segments/points are not your principal aim, and if your itinerary fits, there are other products that can get you around the world comfortably and more cheaply - the United/Emirates North Pacific fare ex-USA is $4850; the El Al/Qantas RTW is $4969 (plus $300 if via Africa)... and there are others.