Donald Trump believes that America has no friends and allies–only economic interests and competitors. He has made it clear repeatedly that he despises the EU, that the Germans are every bit as bad as the Chinese, that the ultimate yardstick for international relationships is our trade deficit, and that NATO should be operated as a protection racket.
These ideas are outside of the mainstream of the GOP, let alone the country as a whole, so the Democratic nominee, regardless of who it is, will reverse them if elected. The absurd steel and aluminum tariffs will disappear. There will be no more discussions about tariffs on foreign cars. Calls for increases in the defense budgets of EU countries will be much more muted. Things will go back to the way they were, with some scar tissue remaining on both sides.
Some trade issues will be more complex. I anticipate that all of the Democratic candidates will oppose the USMCA on the grounds that it does not do enough to protect American labor; there is no obvious political down side to attacking Trump from the populist left. That issue, one way or another, should be resolved before the 2020 election, and I would anticipate that Democratic nominee, if elected, will not seek to relitigate the matter. If, for whatever reason, it has not been resolved, things could get a bit awkward with Canada and Mexico.
There will be calls from Japan, Vietnam, and Australia for the US to join the son-of-TPP. There are very good economic and geopolitical reasons why that should happen. Will it? Unfortunately, probably not, but one can always dream.