Avengers 2? Shouldn't we wait to see if the first one has made any money yet?
An Ant Man movie has been in developement for a few years, and is currently slated for a 2014 release. According to IMDb.com, the script is done and there's already a director attached. Seems likely to me that an "Ant Man" movie would lead directly into the next Avengers film (what other purpose does that character serve?), hopefully setting-up a big battle with Ultron, and maybe introducing The Vision.
From Wikipedia (the internet's least reliable source):
"In June 2010, Marvel Studios hired Thomas Dean Donnelly and Joshua Oppenheimer to write Dr. Strange. While promoting for Transformers: Dark of the Moon, actor Patrick Dempsey indicated he was lobbying to play the title character."
Since it's a Marvel Studios production, I doubt there would be any problems squeezing Dr Strange into an Avengers film. Personally, I'd rather see Strange teamed-up with The Hulk for a Defenders film. Rumor has it that a movie based on Nighthawk is in the works, so maybe The Defenders isn't that far off.
Regarding Black Panther,
from a 2011 article at The Hollywood Reporter: "Black Panther is back in development at Marvel Studios. The company has hired Mark Bailey, who hails from the documentary film world, to pen a take on the character, the king of a resource-rich fictional African country who becomes a super hero. Marvel's Kevin Feige is producing. Marvel reacquired the rights to Panther in 2005 when it absorbed a host of character licenses from Lionsgate, which bought Artisan in 2003."
My guess is that, since this news is over a year old, Marvel wasn't happy with what Bailey wrote, or else we'd be hearing news about them getting a director and lead actor attached, as well as getting some tentative release date.
So, anyway, to actually answer the question of which hero I'd like to see in the next Avengers film, it's The Vision.