A few considerations"
1. Jet lag: I also find it easier traveling west. But that only applies to transatlantic and trans pacific flights with large time zone differences. On an extended journey of a year or more having jet lag issues ofr a couple days shouldn't be the deciding factor.
2. Anticlimax (horse races back to the barn): Once you are half way, or have been to the place that you consider to be the climax of your trip, and you are on your way home, even if there are months left to go, there is a psychological anticlimax to get it over with and to go home at a faster pace. Take this into account in scheduling.
3. It all becomes a blur. If you are on the road a long time (what long constitutes differs for everyone), and see a succession of very different cultures, unless you document each as you go along (that's what Travelblog is for), it can easily become a blur. That's why I like to do individual trips to separate areas as it appears you have done thus far. This way each trip and area is a distinct memory. Certainly, I would do Europe, the Mideast, and north Africa separately as you are from the UK.
And it appears you have done the U.S. adequately, with five fly-drives...the best way to see the States. So one possible itinerary that takes into consideration these cautions would be UK to Brazil, Argentina, Chile, (skipping Bolivia and Peru which you have done, and saving the Caribbean for another trip since it is off the direct route), Tahiti/Fiji/Cook Island, New Zealand, Australia, Indonesia, Singapore, Malaysia, Thailand, Laos, Cambodia, Vietnam, and China, before returning home would be good from the perspective of easy cultural transitions, and leaving the heavy duty shopping towards the end.
If you were really adventurous and still had the travel bug, you could go from China, along the Silk Road through the Central Asian republics, to Turkey, or alternatively the trans-Siberian or trans-Mongolian rail to Moscow, and then home. I am not a big fan of India, but that would be an alternative route home from CHina. If you did India, and still wanted more, then you could stop in Dubai and then swing through Egypt and East Africa. I have always been intrigued by the overland tours from Egypt or Kenya to Victoria Falls and on to South Africa. But that trip I would never put on the end of this itinarary, but do it separately. So you can keep your options open.
One final consideration.
4. You do not want this to be the FINAL trip, after which you will settle into comfortable retirement, dreaming of your ultimate trip. You should always be planning the next trip! So whatever you miss on this trip, you can always go back! This may be the compromise you can reach with your wife...one year around the world and then four three month long trips to various regions.