I'd like the sonic cane to return.
The thing is, Capaldi is a fan of the show, so he might stay on longer than two series. He'll probably stay for three years, so as long as there's none of this split series nonsense, we can expect a full three series from him. But yeah, I doubt he'll stick around as long as Smith or tenant did. In the end, Tennant stuck around for about 4-5 years, if Capaldi did that, he'd be knocking on 60. Age would be another problem for Capaldi had, Tennant and Smith were youthful, so they probably had more energy to commit to the show, and if 30 year old Matt Smith said he was knackered from the show, god knows how Capaldi will feel. I just hope he doesn't go for the same reason Hartnel did.
I also get the feeling Clara will leave at the end of series 8, or at least early to mid series 9. And I doubt she'll be a very good match for Capaldi's doctor.