Luke Skywalker is a long shot to appear in Season 2 of The Mandalorian but he has to be included on the list of characters who could potentially duel Moff Gideon. He has to be included because he’s basically the only known lightsaber carrying Jedi in the galaxy. With The Mandalorian being set five years after Return of the Jedi, we know Luke is alive and well in the galaxy.
He’s either out hunting for Jedi and Sith artifacts or is busy training his sister Leia in the ways of the Force on the planet Ajan Kloss.
The Mandalorian, as stated above, will be on the run from Moff Gideon and could seek out asylum on this jungle planet. Luke would be able to sense Baby Yoda through the Force and that would bring the three face to face to face. Moff Gideon could track the duo to this planet where he would then confront Skywalker in a duel with the Darksaber.
Moff Gideon would be no match for the Jedi Master and Luke would make very quick work of the Imperial warlord. He could then give Mando the Darksaber and decide whether or not to train The Child in the ways of the Force. Again, it’s unlikely the biggest name in all of Star Wars ends up making a cameo on The Mandalorian but it is a possibility that would make sense story-wise.