I don't think Viggo wants another franchise, outside of wanting to do Eastern Promises 2 with Cronenberg. Again, the whole "Viggo as Stephen Strange?" stuff is just conjecture. The "Dr. Strange cameoing in a Phase Two film" and the "Viggo in talks with Marvel" rumors both appeared around the same time. A few sites combined them and it spread like wildfire. I still think it is far more likely that Viggo is once again being looked at for villainous turn in a comic book film, this time for Cap 2....which is currently casting. He previously turned down Batman Begins and Man of Steel (as well as Snow White and the Huntsman and The Dark Tower). On the whole I don't think Mortensen really gives a shit about starring in big films, so don't be surprised if he knocks Marvel's offer back as well. That said, I think he'd be far more likely to play a villain with a two (or three) film contract than the required minimum 6-film deal that all of their heroes are signing.
As fun as it is to speculate who will be playing Strange, I think we'll be doing just that for awhile. A Dr. Strange film isn't going to happen until Phase Three at this point, meaning 2016 at the absolute earliest. I can see Marvel chucking a throwaway little role to a smaller hero or two in the next round of films or on the SHIELD show (like Luke Cage, Iron Fist, Moon Knight, etc.), but I'm not expecting them to do any such thing with the likes of Dr. Strange, The Black Panther, The Sub-Mariner, etc. Those characters, outside of post-credits sequences, likely won't arrive until they have their own films. And expecting Marvel to put Stephen Strange in a post-credits scene and then not have his film come out for two or three years is moronic.
Iron Man 3 will probably tease Thor: The Dark World after the credits. The latter will likely do the same for Captain America: The Winter Soldier and that one for Guardians of the Galaxy (and/or Avengers 2) and so on.
The real big question now is whether or not Marvel is still toying with the idea of squeezing Ant-Man into the schedule BEFORE Avengers 2 hits screens. All rumors still point to Wright moving straight into pre-production on it as soon as he finishes post on the now-shooting The World's End.