Back at the start of season five, I was really hoping the River Song story would be a lot...simpler than what we got. I really wanted River to 'd just be a normal human that the Doctor met out of order.
It would've been interesting if, at some point down the road, they cast a younger River (early 20s) that became the show's full-time companion after the Ponds. That way we don't have to keep worrying about Alex Kingston looking older with every "younger" appearance, and maybe have River meet more than two docs.
We'd get a season or two of witnessing a Doctor that knows everything about this woman, and a girl that knows nothing about him, and watch, in real-time, this relationship flourish, getting a true sense of just how close they were to each other.