Trump acts like he's going to run again, maybe even declares his candidacy in 2021 to freeze the field early. Other candidates still compete in the "invisible primary" (i.e., they do shit like occasionally travel to Iowa or raise money for "the future"). In 2023, we'll know if Trump is serious about running in 2024 or not. It's possible that Trump could have legal issues (from his business, not his presidency - the political class knows not to prosecute one of their own), financial issues (We don't know what his financial situation actually is), or medical issues (he'd be 77) that preclude a comeback campaign. Therefore, even if he declares a candidacy in 2021, it would not guarantee he's a serious candidate in 2023. Likely he is, but you never know.

If Trump runs in 2024, he will win the primary easily and likely unopposed. Anyone who thinks they can beat a man who won 74 million votes just three years prior is an idiot. Plus, all of the "rising stars" in the GOP are young (for a politician). No need to try to challenge Trump when you can just wait until 2028. Maybe some Never Trump stooge with nothing to lose like Kasich or Larry Hogan tries to run against Trump but like GOP primary voters are raving MAGA lunatics so Trump would carry at least 44 states (maybe some new england douchebag wins new england, who cares). Either way a GOP primary with Trump in it is not competitive.

If Trump doesn't run in 2024 or withdraws in 2023, the most likely scenario is the most Trump-like candidate wins. All of the ghouls making moves - Marco Rubio, Mike Pence, Nikki Haley, Josh Hawley, Kristi Noem, Ted Cruz, Mike Pompeo, maybe Ben Sasse(?) - run their little campaigns. Pence will blow the doors off all of them if he runs IMO, because he's most connected to the Trump brand. Sure, he's a human Ambien pill with the charisma of a brick, but he's the heir apparent. He was Trump's loyal #2 for the term, there are like 20 million people who already have Mike Pence's name on a sign, and ultimately I imagine the average Republican voter sees him as the default. That puts the burden of proof on any other candidate to pull votes from Pence, which is hard.

The only person I could see seriously beating Pence in a primary is Trump Jr. because he's the only candidate as tied to Trump as Pence is. So if Trump Jr runs then it's probably either him or Pence. I just think Trump Jr is a lot less likely to run than Pence is.