After the 2020 election, Trump should pardon Assange.
Why would Trump do that? (Serious question)
As mentioned, pardoning Assange will be politically unpopular. The best time to a politically unpopular move is immediately after an election because you are maximizing the time until you need to face voters again.
If Trump wins reelection, and only has a narrow majority in the house and senate, he might wish to wait until after the 2024 election because pardoning Assonge might inspire enough Republican congressmen to be upset enough so they become opposed to Trump's agenda.
Meh, it still doesn't seem like a good reason. Why would Trump pardon the guy, that is definitely gonna keep digging under US government and sabotage his job? On one side we've got some mysterious inspiration for congressmen and on the other side we still get a real threat. I think their choice is obvious.
But at least now we see real accusations instead of claims that Julian raped someone.