There's been no mention, that I noticed, of consolidators for multi-segment air travel. I've had great success over the years using Airtreks and now, after a number of bookings, have a "relationship" with an agent there. The only job of a consolidator is to find their clients the cheapest fares point to point and string them together to make a unique personalized trip. They use airlines with which they have contracts for the lowest fares so, short of a spectacular sale, will, as far as I can tell, always beat the competition, including the airlines' own sites. Sometimes you'll find yourself flying on an airline you've never heard of but it can have the advantage of getting you to places the others may not go.
To give you an idea of the sort of convoluted itinerary one can devise, on my last RTW I started with a flight bought with miles, business class transpacific, SFO to Shanghai. Airtreks then got me to 2 more destinations in China. Train to Hanoi. Airtreks, Hanoi to BKK, BKK to Yangon and back to Singapore. Ship to Malta. Airtreks, Malta to Rome, then Rome to home a couple of weeks later. There is hardly an itinerary you can think of that a consolidator can't accommodate. The ones I've dealt with, 1 other, do only multi-continent trips but there are smaller ones, I believe, that will do a simple RT. I have bought a simple business-class RT (AZ to Delhi) from Airtreks.
For anyone with an imagination and curiosity about a number of places, I recommend looking into using a consolidator. Airtreks has a "trip planner" on their website to use even when you just want to dream. http://tripplanner.airtreks.com