If Marvel were to create a new "Team-Up" book, I'd go with the old standby of ... no ... not Spider-Man, but The Thing! As a superhero who routinely faces cosmic foes, he's tough, but he's not all so powerful that he could be bested by even a weak foe. Also, unlike Spider-man, he has the means and reason to travel the globe as need be. Additionally, he isn't oversaturated like Spidey.

In the DC Universe, I'd go with Captain Marvel. He's tough, but naive at times. He has a mentor that can direct him to places of crisis. Also, unlike Spider-man, Batman or Superman, he's clearly not oversaturated either.
As a back up choice, I think Blue Beetle (even a non-joke cracking one) would be a good choice. He could be directed by Oracle. He currently has all the free time he wants, and he has cool transportation.