1. Series 2 Redux
In a show about time travel, it seems like the the obvious option to bring Ten and Rose back is to bring a past version of them into the present/future. The Doctor and Rose from Series 2 run into The Doctor and Clara from post-Series 7B. And much like in the previously mentioned Time Crash, Eleven knows that he must meet up with Ten because he remembers doing it when he was Ten. Or perhaps the previously mentioned time stream corruption has caused Eleven to have no memory of this encounter. And thus, he does not know how it will be resolved. Perhaps, the special takes place before Army of Ghosts and Doomsday. There can be some mention of this and Eleven can get sad because he knows that is where he lost Rose for the first time.
Likelihood of Happening: 9/10 - The most obvious answer is often the right answer. Why go through a ton of confusing and complicated story ideas when you can just pluck Ten and Rose from their time and bring them to Eleven and Clara's time? And I think that many fans would be upset if The Tenth Doctor and Rose that we got were not "real" Tenth Doctor and Rose. But with Moffat writing the special, he could always throw us a curve ball. What if Ten and Rose are Teselecta ships? Or Flesh creatures? Maybe the 50th anniversary is just a hallucination brought on by the Dream Lord? Only time will tell. The Doctor Who 50th Anniversary Special airs on November 23 on BBC One and BBC America.