“It’s still early days, but I told [Whedon] after reading the first sketch outline he delivered — and there’s a lot of work still to do, he has to sit down and turn this into a movie — but there are already seven things in there that are worth the price of admission alone.”
Fiege could not reveal much more than that, but it could be speculated that he could be referring to seven Avengers involved in "Avengers 2".
Joss Whedon has always been a strong proponent of utilizing strong female roles in his creative works, this this could lead to the idea that Wasp would be a likely candidate. And Whedon did mention that in "Avengers 2" there would be no exception.
Six team members were in the original movie, so it would stand to reason that a seventh would be likely a female. Also, one could consider likely replacements to the team. For instance, Hawkeye and Black Widow might be considered expendable but since Whedon prefers keeping women characters in the mix, Black Widow could stick around. That being said, Hawkeye would be the likely candidate to be "voted off the island".
So, if Hawkeye were to be replaced or moved to a lesser involved role, who would be there to replace him? Considering Ant Man is coming out with his own movie on November 6, 2015, he could move in to replace Hawkeye. The justification for this motive would be to introduce Ant Man in "Avengers 2". This could then build momentum for this not so well known character to be more accepted in the "Ant Man" movie.
On the other hand, Disney might want to either introduce Ant Man at the end of "Avengers 2" in a teaser segment at the end or perhaps save the bulk of his introduction and origin in "Ant Man" altogether.
So who would be next after Ant Man? Other candidates could include Black Panther or Falcon. Falcon is to be introduced in the "Captain America: Winter Soldier" movie, so a singular movie wouldn't be necessary for him, while there are rumors going around that Black Panther could also be a candidate.
The basis by which the movies might introduce these characters run in parallel with how they came into being in the original comics. So one can base this speculation on that reference.