If you are yet to own the 16-35 you might want to consider the 18-35mm f1.8 Sigma.
Much better image then the 16-35. Better feel on the focus. Longer throw on the focus. Faster aperture.
I'd avoid Canon lenses on the pocket tho unless you are gonna use an adaptor that powers the aperture (like the RedRock Live Lens) as the cheap adaptors with built in apertures have the aperture in the wrong point of the light path.
You could get the Nikon mount Sigma and Metabones Speedbooster and Metabones standard adaptor. You'll then have two lenses in one.