A few things to start with.
1) I am more of a DC fan than a Marvel fan
2) The Next Avengers are the exception that proves the rule
3) THIS IS MY OPINION
Time to begin.
Marvel has already introduced most of the necessary characters (Iron Man, Hulk, Hawkeye, Black Widow, Captain America, Thor) and are introducing others soon (Ant Man, Wasp, Ultron, Vision). These pre-existing characters mean that little preparation is needed the movie to work.
Free Standing Film
Next Avengers doesn't need to have any actual connections with the MCU, because it's set in the future and could even be in an alternate reality.
A Darker Tone
Next Avengers would be hard to do with a positive view. A darker tone would fit better with the story (most of the Avengers die and humanity gets enslaved by Ultron).
Next Avengers would be a great way to bring Coulson back to the big screen. Phil and his agents could help lead the underground resistance that helps the Next Avengers fight off Ultron.
I think the Young Avengers would be a great film to see but I don't think it would fit into the pre-existing cinematic universe (which sucks). If we want a film based on teen superheroes then we have to use the Next Avengers.