As you all know, Ursena is the final boss of Season 2, as well as a heroin which you can summon through Atlantis Summon.
If this is the case, I strongly also believe that the final boss of Season 3 will also be a playable hero.
So why not the Dark Lord? He has a very cool design, I know that he shares the same special skill with Sartana, but they can always change his special skill. As many playable heroes that we fight in Season 2 and 3 also have slightly altered special skills.
It also can be a last hurray for Season 1 too.