I'm not sure how you're planning to increase the likelihood past its
current 'certainty'.
Iron Man III, Thor II and Captain America II were all already greenlit and
in pre-production, before the Avengers. Disney has announced Avengers II
to a shocking lack of surprise. The original financin structure for Marvel
Studios was for 10 films. Some of the original titles on the list (such as
Power Pack) have been non-starters, but as of now, the studio has produced
6, and now has 4 more green-lit, which will complete the original
financing.
Primarily Avengers related superheroes who might be considered (and whose
rights aren't currently otherwise optioned)
Ant-Man
Wasp (probably the most obvious potential additions)
Swordsman (unlikely, i think)
Black Panther (he was one of the characters specifically listed for
development by Marvel Studios)
Vison (probably 2nd most likely after Hank and Janet, likely without the
Ultron baggage)
Black Knight (slightly more likely than Swordsman, but not much)
Wonder Man (down the line maybe)
Hellcat (much as I'd like to see her, not likely)
Moondragon (with the personality without the baggage)
Ms. Marvel (not sure without the Kree stuff)
Falcon (could work well)
Tigra (could work)
Shulkie (I don't see her introed directly in Avengers)
Captain Marvel (Monica Rambeau) probably not as CM, but couild work under
one of her other aliases
Starfox (makes sense if they go with Thanos, still, not likely)
Doctor Druid (if they go that way, they have the rights to Dr. Strange)
Mockingbird (has a tv pilot in development at ABC Family)
Firebird (a Catholic based superhero?)
D-Man (no, except as a one-off)
U.S. Agent (no)
Quasar (hmmm,)
Stingray (could work, but primarily water-based)
Eternals (various Eternals, including Gilgamesh and Sersi have served, I
don't consider any of them likely to appear in the movies)
Lionheart (is the Captain Britain stuff too X-ish)
Luke Cage (it would make Bendis happy, but Cage is supposed to have a role
in the AKA Jessica Jones tv series in development at ABC)
Echo (I could see her working well, but are her rights tied to the
Daredevil franchise?)
There are a few others (Deathcry, Silverclaw, etc) that could be
consisdered.
--
I have a theory, it could be bunnies