My best guess is that he'll be dressed nothing like the last 3 Doctors.
I don't think we'll get one looking as casual as Nine for some time.
Put Capaldi in a suit, even one as quirky as 10's, and he'll look too Malcolm Tucker.
11's outfit was perfect for Matt - the old soul in a young body - but if you put an older actor in it, he's going to look like a university lecturer.
While one might say, just go for the opposite of Matt - put an older actor in a younger man's costume - that's a tricky one to do. Mutton dressed as lamb and all that. Older men dressing like young men frequently look like knobs. IMHO. So my best bet, which will probably be wrong, as usual, is that they'll put him in something flamboyant, like Pertwee, Tom Baker or McGann. Hopefully nothing as frankly silly as Colin Baker's though.