1. He's Got A Definite Mission
BBCSo where is Gallifrey? We know it's somewhere, in another universe, in a painting, in a cube, it's all a bit Sci-Fi. The Doctor now appears to have a clear goal set in front of him: to find Gallifrey, whatever it takes, and to restore it into our universe, whilst being careful not to restart the Time War. Back in the Tom Baker era, the Doctor had a mission, spread over a whole series, to find the Key To Time. It reinvigorated the show along with the Doctor, and the hunt for his home planet is bound to have a similar effect. He's got a lot on his plate then, but, as our favourite time lord flies randomly around the universe, he knows he's always got another adventure to look forward to, possibly the biggest adventure of his life. When the Doctor eventually does find Gallifrey and manages to restore it, we can be sure that he'll be the happiest he's ever been.
To Conclude...
BBCAlthough many people may wish to see Peter Capaldi playing a darker Doctor, it may not make sense for Capaldi to be dark from the outset. An alternate route would be that the 12th Doctor could become darker as his series progresses, maybe as he experiences more shocking events or has to battle even more terrible creatures. All we know for certain is that Capaldi will completely knock it out of the park, and hopefully take the place of the coveted 'favourite Doctor' position in many peoples hearts. We won't know until August whether or not the 12th Doctor will be more menacing than his predecessors, but there is substantial evidence to suggest that he shouldn't and that he should remain the jolly traveller that we all love.