The Hero could beat Solas easily, depending on the build. Even the weakest Hero could beat Solas. However, I think Solas is not worth the Hero's time. The Hero should be saved until the villain who is using Solas as a puppet.
I say just let the (ex)Inquisitor and the new DA4 protagonist handle Solas, but also use DA4 to ease both the Hero and the Champion back into the main storyline.
Logically, the Hero must return in some capacity, even if dead. At most, the Hero is the King of Ferelden, and is thus a major political force on the world stage. I think this is partly why they demoted Hawke in Trespasser. And at the least, the Hero is an honored figure in the halls of Weisshaupt, with his or her own tomb next to Garehel's. And it's likely that we will see Weisshaupt in DA4, so its only natural we visit the Hero's tomb, which would naturally be designed to reflect the Hero's life and identity.
DA:I needs one more DLC in which the Hero or, if dead, the Orlesian Warden has their storyline featured. The DLC would cover the events between wherever they were before the quest began (saying goodbye to any loved ones), all the way to the discovery of the cure (if it was discovered). This would obviously begin with the character creator, and this would allow the players and BioWare to save the Hero's appearance and skill set for the next game. That way we won't have to stop the game to create a returning character, like we did with Hawke. Everything will be saved and ready to go in the next game. The DLC would also feature possible returning companion characters, such as Oghren, Sigrun, Velanna, Finn and Ariane (who are a package deal now I've read), and definitely some new characters. I would like to see some people of the far west join the party and get to know their lives and cultures.