I'm quite certain that the world will still be around after 6 more years of Trump, and fairly certain it won't contain much more cesium-137 or strontium-90 than we expect from normal reactor operations.
I'm also fairly certain that the formal rules of the Constitution won't be thrown out. Even if he avoids a lethal heart attack by cheeseburger overconsumption or anything, Trump won't attempt to stay in office past the end of his term or suspend elections. That's not how electoral authoritarianism works - you trample every other liberal democratic norm, but elections themselves happen reliably and the letter of the Constitution, as interpreted by the reliably conservative SC, is followed. But I will sketch out a scenario that I think is roughly what the future would look like.
Voter suppression is ratcheted up in many states by striking any non-voters off of voting rolls following a failure to vote even in a midterm, and early voting is ended in several states that currently allow it. Gerrymandering becomes even more extreme following the Republicans' strong showing in 2020, with sophisticated algorithms used to create a map resembling
this in about half of all states. Democrats counter, but they still only hold 12 state governments outright following 2020 and can't come close. With the similarly gerrymandered state legislative districts, they're not likely to hold much power in 2030 either.
Secure in their 7-2 majority following the deaths of Ginsberg and Breyer, Roe v. Wade is chipped away at in a series of rulings that don't quite overturn it but allow so many arbitrary restrictions that all the clinics have closed in many states with GOP control.
Grants to states for welfare, education, environmental protections, etc. are slashed and requirements for welfare are tightened to the point where most people who need it cannot jump through all the hoops necessary.
Trump's popularity nosedives in 2022 as the Iran War goes miserably and the market crashes, triggering a new recession. The Dems manage to narrowly carry the Senate, 51-49, and come within 10 seats in the House despite the extreme gerrymandering. This does allow them to reject some of the stuff he tries to do in the last two years of his term.
Global trade falls and average tariffs increase following Trump's actions, but the economic effects are smaller than anticipated worldwide and most other countries do not impose tariffs against each other, sticking mostly to retaliatory tariffs against the US. Even the Chinese recession following the US market collapse in 2022 seems to have more to do with financial contagion and the high amount of bad debt than to its decreased exports to the US.
The invasion of Iran, coordinated with Saudi Arabia and Israel in 2021, meets fierce resistance and takes four months to topple the government. Once in nominal control, Shia militias wage an effective guerrilla war several times the scale of Iraq, with plenty of mountainous terrain to exploit. A fierce campaign of airstrikes, mostly by drone, results in more recruitment than militant deaths. By 2024, the American death toll has reached 20,000; the Iranian one somewhere above 600,000. Our allies for the war turn out not to be very helpful either, especially following the Islamist coup that overthrows the Saudi royal family in late 2022.
Trump's relations with the rest of the West improve somewhat, mostly because right-wing nationalism becomes the dominant political force in much of Europe. Trump remains unpopular internationally and the leaders try not to be seen to be supportive of him, but Trump gets along better with them all the same. By 2024, most of Europe is ruled by parties or coalitions with substantial right-wing populist elements. Macron is defeated in 2022 and replaced with an ardently right-wing LR president, who wins in part by shifting right and getting votes from FN voters; Marion Le Pen gets barely half her mother's total. Merkel chooses not to run in 2021 as the CDU is taken over by people with considerably tougher stances on immigration and multiculturalism; they nearly gain an outright majority Bundestag as AfD contracts in those elections and easily govern with the FDP. Trudeau and the Liberals are re-elected in 2019 but are forced to rely on the NDP in a confidence-and-supply arrangement; this fails during the 2022 recession, and Doug Ford's Conservatives win the ensuing election in a landslide. The only large country to visibly shift left is the UK, where Labour wins easily following the Brexit debacle and Corbyn becomes PM.
Relations with North Korea go through cycles of threats/missile+nuke tests/etc and friendly periods; the status quo more or less remains. Relations with Russia continue to be chummy, and Trump eagerly follows his suggestion for funding for human rights organizations to be cut. Freedom House downgrades the US to "Partly Free" - why not tell the truth now that it's no longer being funded? Relations with China continued to be strained because of his erratic tariff policy; the incident in 2020 where he agreed to lower tariffs only to raise them in response to a perceived slight on Twitter was especially problematic. But there is nothing resembling a military conflict.
The Democrats do finally win the Presidency in 2024; Trump's approval rating remains at 30% as the "recovery" from the crash in 2022 grinds on. They even manage to win both the House and Senate narrowly. They'll lose the House in a landslide in 2026 thanks to their gerrymandering disadvantages, and practically nothing gets through the Senate due to the filibuster. But it will be fun while it lasts.