To me, the Canon EF 24-70mm f/2.8 L USM II makes the most sense. It's an excellent general purpose zoom - and that seems to be where your current kit is lacking. The EF 70-200mm f/4 L USM is a fantastic lens, but there are many times when you need a good zoom lens below that focal range.

The 24-70 should come in very handy during weddings, especially with a full-frame body that handles noise as well as the 6D does. While primes are fantastic for weddings, they can be problematic if you aren't carrying two separate bodies on your person at all times. Wedding moments are so fleeting that it pays to have a single lens that can cover a wide range of situations without having to change lenses.

But jrw is right - figuring out what you want to do but are currently unable to do with the gear you own is the best way to determine exactly what you need. :-)