Trump
Strange as it seems, we still have - years later - controversies and disagreements about whether there was collusion between Russia and the 2016 Trump presidential campaign. It seems like this would be a good toic to get on the same page about before the 2020 presidential election, as there must be tens of millions of Americans who are laboring under delusions and won't be empowered to vote responsibly unless we can clarify this topic.
Voting

Republicans have long complained that voter fraud has been perpetrated and that this has les to losses on their side. It has even prevented some forms of mail-in voting (although absentee ballots are still accepted), which is a particular issue during the coronavirus pandemic, and may still be an issue for the 2020 Presidential elections.