One of the main reasons on why Doctor Who is so fantastically great, is that its constantly changing. All the time. Especially with each incarnation of The Doctor; a different persona, look and feel after every regeneration.
Therefore, with a brand new era beginning for the Time-Lord, Steven Moffat certainly wants to make sure that the Twelfth Doctor will at least try to be something of its own, and as far from Matt's or David's Doctor as possible. In the latest DWM, the head writer gives us a small insight on what he plans for the Capaldi's time as The Doctor:
“I think you can be a little more ambitious and dangerous with your new Doctor if you've got an established world into which he can bed.”
“There would be little point in making as radical a change as we’ve made unless you’re going to go quite different with the Doctor. The last two Doctors have been brilliant, and have been your ‘good boyfriend’ Doctors. But the Doctor isn’t always like that. There is the sort of Tom Baker, Christopher Eccleston end of the spectrum, where he is mad and dangerous and difficult.”
“If we’d cast Ben Whishaw, you’d know what we were doing. You’d think, ‘Well, that’s another quirky young man with entertaining hair!’ And he’d be a brilliant Doctor, but Clara would just think, ‘Okay, you’ve rearranged yourself a bit…’”
“I think it was time for the show to flip around a bit. The new version of the show is quite old now. It’s very old. And just because we've been reminding them [the audience] so vigorously that there were other Doctors - and they'll have just seen the John Hurt version of the character - you can actually say that he doesn't need to be like David and Matt were, and I think that's just exciting.
“We need the kick-up-the-arse Doctor, in a way, to frighten you and make you think, oh, it’s a different show again.”[Source]