Its not about new villains/heroes, it's about making the already existing ones, cool. They made Star-Lord a house hold name, and he was a huge nobody both in the comics and in television. They needs to make Doom cool again, both movies dropped the ball, hopefully some director out there makes Doom a force to be reckoned with again.
As for heroes, that's already handled. Avengers did it beautifully, then again that's what Joss Whedon does, he makes the movie about the group, he focuses on the chemistry, the camaraderie.