I would never be away from home for a year. For one thing, we have to file our income tax every year, usually do that in February.
Hypothetically, for someone that hasn't traveled a lot (I have visited 78 foreign countries and been to every continent except Antartica), I would break it down this way.
For an American, I would save the USA for future domestic travel. Also, Canada and Mexico.
Limited to 90 days in Schengen zone, but there is lots to see outside that zone.
Start with the British Isles. I would spend a month in England, Wales, Scotland and Ireland.
Also, we love Norway (take a cruise up the coast to the North Cape) and Denmark, spend a couple of weeks in those countries.
For the EU area, Germany, France, Switzerland, Spain, Portugal, Italy you can easily spend a month in each country, but can't do that, so don't miss Paris, Normandy and Provence in France, a week in Belgium and Holland, three weeks in Germany with one week in the Rhineland, 10 days in Bavaria and 5-6 days in Berlin. Switzerland is expensive, but a week, start in Interlaken then Lucerne.
Spain, three weeks don't miss Madrid, Segovia, Toledo, Seville, Granada and Barcelona. Portugal, Porto and Lisbon with lots in-between. Italy, do a month, with Rome, Florence, Venice, Naples Area, Ravenna, Sienna, Lucca, Perugia and if possible Milan and Verona. Croatia, go with Split and Dubrovnik. Greece, Athens and surrounding area, plus several Greek Islands, especially Santorini.
Istanbul in Turkey. I love Poland, Czech Republic and Hungary, so plan on a couple of weeks there.
Don't miss Egypt and a Nile Cruise as well as Israel and Jerusalem. Russia and Ukraine are great to visit, but save them for the future.
India, China and Southeast Asia all merit visits, but would recommend tours, not doing it on your own. Australia and NZ, if they are open to visitors. Plan on a month for both. Japan, which you can do on your own, don't miss it. The people are wonderful and it is the cleanest country in the World. Lots of history in Kyoto and Nara.
South America, our favorite countries there are Peru, Chile and Argentina. SA is cheaper and each country is different. Tours are best done in Peru, don't miss Lima area and especially Cusco and Machu Picchu. Chile and Argentina are best done with a cruise around the Horn. Spend 5 days in Buenos Aires and 3 in Santiago.