Marvel Studios President Kevin Feige said at the CineEurope event in Barcelona this weekend that Doctor Strange will be Marvel’s movie foray into a third, supernatural, area of the Marvel universe, joining superheroes and science fiction. 
"I believe it's very, very important for us to explore every nook and cranny of the comic book universe,"  Feige said, according to Total Film.  "We've done the street-level heroes, and we've done the billionaire superheroes.  We've now done the cosmic side...  But there's a whole other side of the Marvel comics, which is that supernatural side--the interdimensional side of the Marvel universe.  And Doctor Strange is, I believe, our entry point into that other realm, which has dozens of characters and storylines all of its own."
For Doctor Strange, Marvel plans to cast the main character: "in the next month or two," with plans to begin movie production in Spring 2015.  He stated that "Strange is very, very important, not just because it's an amazing character study,... but it also is going to open up a whole other side of storytelling for our movies "
Scott Derrickson has already been named as director(see "'Doctor Strange' Director Set"), and Marvel Studios is in negotiations with screenwriter Jon Spaihts for a script redraft (see "'Prometheus' Scribe to Adapt 'Doctor Strange'").
While Doctor Strange could open up the supernatural side of the Marvel universe, Guardians of the Galaxy is positioned as a touchstone for intergalactic entry.  "...[T]there are dozens and dozens of Guardians of the Galaxy storylines - there's a group that came about in the '60s, they were revamped again in the '80s and they changed into this incarnation that you see in this movie about eight or nine years ago - so there are a lot of other characters and worlds and stories.  So should this film work, should the audience come out for this one, there are definitely places we can take it and we have ideas of where we'd like to go..."
Feige also pointed out that the appearance of Thanos in the first Avengers film  is what ties the sci-fi and super-hero parts of the shared universe together.  "We see him again in this movie [GotG], we learn a little bit more about him in this movie, and he - and his band of followers--is the biggest piece of connective tissue that will eventually lead us back into Avengers films in the future."  Those connections flow through all the Marvel films, and also through its TV shows, including the upcoming Netflix series, according to Feige.