Some possible scenario I can think off for rigging election
1. They create a situation where martial law applies (blaming riots), send in the national guard and freeze election
2. They plant some prove of some Mail in voters fraud, demanding a recount / reelection or possibly demanding the SC to declare the election as fraudulence. Every dispute ended up at court eventually
3. Trump could start a war with Iran/China or Russia and use it as an excuse to hold election (looks improbable but doable)
4. Declare the pandemic to be widespread and only mail in counts in certain areas (according to his interest)
5. Reject the outcome of election, citing fraud and asking the SC to decide (happens in my country btw)
6. Delay Mail in Voters tactically, quickly do a quick count and claim he won. Or something like that, early results would stop late voters to participate.
7. Outright closing voting booth discriminately
8. Get some dirt of Biden and use it as last minute blitz. Hillary's email 2.0
None of them is plausible normally, but it's never normal with Trump, he really has everything to lose