Hey I'm back!
So, after a very long run of adventures, our Fourth Doctor (Rory Kinnear) would eventually bite the big one. I imagine him sacrificing himself in a very non-meek and yet simultaneously humble way to save a planet or a galaxy of some sort in a blaze of glory. After a very sad and mournful death scene, where his companions say their goodbyes, he regenerates into Doctor number five: Ben Miller!
I see the fifth Doctor as a pulpy, dashing space adventure hero, complete with space-suit and jet-pack. After having spent a lot of time on Earth, he'd be aching to see some other worlds again, and immediately jet off in search of adventure. Flash Gordon meets James Bond, with a little bit of Booster Gold (his companion, or rather side-kick, would be a robot drone called Demo), the fifth doctor's adventures would be the most epic-looking. If we imagine his fifth regeneration as part of the full twelve, you could say he's having a sort of mid-life crisis, and is eager to have as much fun and as many thrills as possible.