Build the wall.
Here is a column by Ann Coulter that lists the number of times Trump caved on building the wall:
Now that we know Florida Gov. Ron DeSantis is running for president (along with several others whose names I can’t remember), I have a helpful primer on what NOT to do as president. I base this advice…
anncoulter.com
But, I don't need her to tell me that Trump promised to build the wall and never did. Memory and current events are all I need.
Yes, I think this is the most likely motivation for DeSantis. Trump is old, fat, and has the weaponized federal government and some weaponized state governments coming after him. He could literally drop dead at any moment to the suprise of no one. He's exactly the type guy that would run at high speed right up until the moment he is suddenly dropped by a heart attack or stroke.
A legal barrier to running is far less likely, because I don't believe that is the true aim. They just want to interfere in the 2024 election the same way they interfered in the 2020 election, by keeping him under a cloud of endless investigations.
I said above, I think that is what Tim Scott and Nikki Haley are doing.