If Trump had won re-election, we would have witnessed the first time a Chief Justice swore in a president whose impeachment trial he had presided over (twice).
Tbf, if Trump had actually won re-election, then there presumably wouldn't have been a 2nd trial on account of there having presumably been no need for there to be an insurrection for Trump to have incited in such an alternate timeline.
It's hard to know whether there would have been a second trial for something else. Trump was insanely corrupt and may have broken norms so badly that Dems would feel the need to rebuke him again, especially in 2023. However, a 2020 Trump win and the lack of a Capitol Riot would make it impossible to even hope that the Republican establishment would turn on Trump so maybe Dems only impeached him because they thought there was a chance to actually score a conviction in the Senate. Trump winning after being impeached the first time may imply that it backfired too, though oddly there were very few of those takes despite the overperformance (or more accurately bad polling).
