While one might say, just go for the opposite of Matt - put an older actor in a younger man's costume
And now I have an image in my head of Capaldi wearing baggy jeans half way down his ass.
Consider, When we watched Tennant morph into Smith, he still wore his pinstripe suit and trainers. It wasn't until the end of "The Eleventh Hours" that Matt donned his tweed jacket and bow tie. Of course, intrepid fans with camera phones managed to snap candid photos of Smith in his final ensemble a few months before the episode broadcast.
Actually, BBC released an official image of Smith in costume in July 2009, five months before we saw Tennant regenerate. Since filming of season 8 is supposed to start in the winter, that's when we'll get our first look at Capaldi's outfit.