Okay, by coincidence I've just finished watching the River Song episodes again and in fact there's nothing in either of them that makes it essential for the Doctor to be played by David Tennant if she ever appears in the series again. Yes, she refers to how he looks a couple of times, but what she says is ambiguous at best. In "Silence in the Library" she says:
"Going by your face, I'd say it's early days for you, yeah?"
and a few moments later, in the same scene:
"Look at you. You're young."
Both of these lines could easily be taken as meaning that when she knew the Doctor, he'd regenerated and no longer looked like David Tennant. After all, if she knows the Doctor so well she wouldn't need to know what he looks like to recognise him: here's this guy in the library which is supposed to be deserted, bounding around trying to save the day and who obviously knows more about whats going on than anyone else, and she's called the Doctor to come help - so who else would it be? So basically, if Stephen Moffat has anything further planned for the Doctor/River Song relationship, David Tennant leaving the series shouldn't affect it. And actually this seems more likely after repeat viewing than I'd previously thought: she refers to "the crash of the Byzantium", and also to the Doctor turning back armies, so maybe their relationship was more action-packed than I previously surmised.
And yes, I know I got the name of the second episode wrong in my previous post. But I like my title better. And henceforth I shall also be referring to the episode "Blink" as "Blunk".