This is GORGEOUS. And it works very well with what we know about River so far in canon; don't worry about that. I'm not usually a huge fan of young!River/future!Doctor stories, but this was utterly brilliant. It felt like THEM. And there were so many lines to love in this. Like this one, "He laughs, tickled; the Doctor, with all the wonders of space and time at his disposal, thinks that there is nothing more thoroughly splendid than a drink that changes color." Such a good insight. Also, the kiss is reminiscent enough of their kiss at the beginning of the series to have symmetry but not to be repetitive. And the way you have this Doctor talk is wonderful, individual enough to sound like a different incarnation but still recognizably enough the Doctor. And then all the little comments he drops that link in to what we already know, such as when River says that he knows better than to fight with her and he replies, "Yeahhhh, that development's going to take awhile." And then the fact that River is jealous of herself a bit and the Doctor misses her even while kissing her. Again, symmetry and beautiful counterpoint to what we have already seen and know about them.

This was just brilliant. Definitely adding it to my LJ memories.