I'm really excited about seeing The Next Doctor...the Dicken's theory would be great if it comes out like that and I agree would make an excellent fan fic if it doesn't. I'm not really sure about what to make of this other Doctor. This whole Tennant leaving after the specials thing messes up Silence in the Library and River Song's appearence, she clearly recognized the Tenth Doctor as being younger and even indicated that he hasn't met her yet. She examines his face...what does the eventual coming of the Eleventh Doctor (if it happens in The Next Doctor or not) do to this story and the prospect of bringing (or introducing) River back?
Nothing has really been "messed up." (I imagine future showrunner Moffat knew when Tennant was leaving as he wrote the story.) There's a gap between "Journey's End" and "The Next Doctor" where the tenth Doctor could have further meetings with River Song, and there will probably be further gaps between some of the other specials. Moffat has said in an interview that River Song has met other Doctors than the tenth, so clearly there's potential for her to be brought back if Moffat wants to. Which he might not; the character really wasn't introduced with plans to recur.
River could easily fit into one of the specials to show how they met or even if she's just mentioned. It's not going to happen - but I wouldn't mind if it did... ...and after months of waiting it feels a little odd that the Next Doctor is on in just over four hours. I may watch Remembrance before it starts