Han Solo is arguably the only Original Trilogy main character not to appear in the Prequel Trilogy.
Does anyone know if there was ever any plan for Han Solo to appear in the prequels?
Actually, yes. There were initial plans for Han Solo to show up in Revenge of the Sith.
The book The Art of Star Wars Episode 3: Revenge of the Sith includes concept art of a young Han Solo and two blurbs about the original idea.
Concept Artist Ian McCaig explains:
It’s not in the script anymore, but we were told that Han Solo was on Kashyyyk [in 'Revenge of the Sith'] and that he was being raised by Chewbacca. He’s such a persnickety guy later on – he always has to have the best of everything – so I thought it’d be great if when he was a kid, he was an absolute slob.
and it provides a dated script note
January 10-24, 2003
Lucas asks for a conference room on Kashyyyk - and a costume for a young Han Solo, who is slated to meet Yoda on this planet and actually help him to locate general Grievous. His one and only costume and character style are approved as soon as they're presented.
Solo was originally scripted with one line of dialogue. The draft with this line is included in The Making Of Star Wars Revenge of the Sith.
I found part of a transmitter droid near the east bay… I think it’s still sending and receiving signals.
Good. Good. Track this we can back to the source. Find General Grievous, we might…
It's worth noting that this concept was not exactly a new one. The Annotated Screenplays (published in 1997) mention this idea even back then.
George Lucas: "The idea was that Han solo was an orphan. He was raised by Wookiees, befriended Chewbacca, and they went off."