If you take a look at high-profile unproduced movies from the nineties (think the Tim Burton/Nicholas Cage Superman Lives) you'll notice a disproportionate number of them were set to include a giant mechanical spider at the end. The world wasn't going through an arachnid obsessed period; producer Jon Peters just really wanted a film with a big spider finale (each to his own) and pushed it on whatever movie he could. The relevance to Who is that this is the exact same approach that Steven Moffat has been taking with his arcs (the pushing ideas, not the spider). Before David Tennant decided to leave the show Moffat was set to take over the show as lead writer. So rather than a new creative head and Doctor arriving at the same time, Moffat's first series was actually going to be Tennant's last. Being the time-twisting storyteller he his, naturally Moffat dreamed up a complicated plot; the Doctor would meet the new companion when they were a child appearing injured, then they'd meet once she was all grown-up, with the finale revealing the Doctor the young child saw was him just before regeneration. When some people hear this plot idea they're curious to see if it'll ever make it to screen. More observant viewers will notice that we've already seen it many times. Amy met the Doctor as a kid and joined him once she'd grown up, River Song met the Doctor totally out of sync (and once when he was dying) and the Doctor kept meeting Clara when she was near death. Heck, the idea had already appeared in previous episodes; The Girl In The Fireplace from has the exact same central idea, minus regeneration. They've been brought to life with a sense of fun, but it's clear the main focus is the idea rather than its relevance to the characters.
