I was just thinking the other day what lenses I might want in my collection besides what I have. Here is the results:
Currently own and use:
Zooms:
EF 24-105mm f/4L IS USM
EF 70-200mm f/4L IS USM
EF 100-400mm f/4.5-5.6L IS USM

Primes:
EF 28mm f/2.8 IS USM
EF 50mm f/1.4 USM
EF 85mm f/1.8 USM

Missing:
*EF 16-35mm f/2.8L II USM ($1,699.00)
*Extender EF 1.4X III ($449.99)
*EF 100mm f/2.8L Macro IS USM ($949.99)

Not sure which is going to come first, This is all for my 6D, so with owning a 7D as well the extender isn't that important. I am thinking the Macro is my most likely candidate.