I doubt we will ever, ever see River Song. I think that was just a character idea that Moffat introduced for one story. It was a concept character....the future companion/lover/wife. Nothing more, nothing less. We can assume he meets and travels with her either between this finale and the Xmas special (how long is he by himself? Could be centuries of adventures), or she's from a future incarnation (which is what Moffat was implying, per the writer himself in an interview in the Confidential) and we will never see her at all again.

As for Donna (Spoilers for Uschi!) I think her ending was the most tragic of any character he's introduced. So, I loved/hated her ending.

As for the Specials, I've heard that it will be one-off characters, different for each story. Also, RTD isn't writing all of them. He has stated that someone else will be writing at least one of them. Probably Moffat, to help the transition I'd imagine...