This is an important element to decide before you start thinking of your itinerary. The majority of world tours last between 6 months and a year (which is often the limited validity of a world tour ticket).
This might depend on your professional flexibility (how long can you leave your work/ studies?), on your economical capacity (how much were you able to save?) and on your willingness to be away from home and family for a long time. If you decide to go with a minimum budget you might need to avoid Japan, and if you go only for 6 months you may need to focus on just 1 or 2 continents.
Some people might do a world tour in different segments, continent by continent for instance, going back home in between. Six months might be better in terms of ease of return and budget and administrative feasibility, but it might also be too short for a complete world tour. One year implies more organization, money and time away but would allow you to achieve most of the things on your bucket list. Some people choose to be on the road for a few years in order to take their time, and although it sounds great, I do believe that the longer away from home the more difficult it will be to come back and re-adjust.
Of course you could also decide to leave an open-end return, but then you might want to switch to a nomadic lifestyle and look for ways to sustain yourself on the go.