Full frame super zooms are a rare beast - it's odd that the only L option is the silly sized 28-300mm which isn't really a walk around lens - only alternatives are older stuff like the 28-135 mm which isn't really a wide a range as what's offered on the EF-S range, where you get the 18-135 and 18-200 (which is like having 29-216 & 29-320mm in your hands)
I'd say that there is a market now with the entry level 6D for a range of L superzooms, giving for example a 20-135mm, 24-200mm (black) and continue the 28-300mm product, probably all retaining a variable f stop, with prices at say £600-800 range for the two newer models, maybe with last gen IS ?