Trump becomes the Republican nominee but is physically confined to a prison cell for the duration of the election
Kamala wins, simply because she is able to go out an meet voters. Trump would get an initial boost in popularity that would taper off as he is out of the spotlight.
Very unlikely though. Even if sentenced to prison, there were still tons of legal moves to keep him out, and SCOTUS would likely be sympathetic.
Popular answer would be DeSantis gets the nom, but it's hard to say as many Republican contenders backed off with Trump's return. DeSantis is an amalgamation of old-school GOP + MAGA, acceptable to both types. I do wonder of Josh Hawley throws his hat in the ring as the "pure MAGA" young populist, a current role filled by JD Vance, who would not have the experience yet to run in 2024. The neocons are not totally discredited yet, so MAGA could lose the primary to Nikki Haley.
DeSantis is stronger without Trump running. He won't need to try to run to Trump's right on abortion and make a restrictive Florida law. It would have definitely hurt if he actually got the nom.
For Trump, he is headed for prison with no chance to pardon himself, so it is unlikely in any scenario he sits it out. He would have had to die before the primaries begin for this scenario.
Trump runs as a third party candidate
Super unlikely in any alt history. Democrat automatically wins if it happened.
The Trump assassination was successful
MAGA dead. Nikki Haley is second in delegates and gets the nom. And she wins due to Biden/Kamala being bad like our timeline + people who liked Trump that want revenge on Biden admin for letting him die. Very shitty timeline.
Trump wins the popular vote but loses the election
Libs find a newfound appreciation for the electoral college. Calls for it to be amended away end for a generation. Shitty Kamala timeline starts, claims huge mandate even with a popular vote loss and floods the country with migrants.
Trump landslide commences. Biden is unable to campaign while Trump is front in center. We see red NY here.
