Anything zoom with f/2.0-2.8.
They've done variable-aperture L zooms in the past, and it's a bit more realistic to expect a f/2.0-2.8 than it is an f/2.0 constant zoom.
I'm thinking something along the lines of EF 50-150 f/2.0-2.8 L, *the* ultimate portrait zoom, that on a 5D3 would make an excellent wedding-kit (IS or not, i'm presuming with IS it would be *huge*, it it's not already).

Any long zoom. The 200-400 f/4 seems nice enough, if not expensive.
There used to be an FD 150-600 f/5.6 L zoom (the first internal zoom ever, iirc). Bring that back with EF and AF and price is maybe somewhere $7k ish and i'd be happy (I couldn't afford it, but i'd sleep better knowing it existed).
Or the Tamron 300-800, make something to compete with that. (400-800 f/5.6L would be nice?)

Portraiture is also nice, there's nothing between the 85/1.2 and 135/2.0, except for the 100 f/2.8 macros and f/2.0 (which is a nice lens, but 20 years old and not exactly L). 100mm f/1.4 L maybe? A new 135 would be nice, 135 f/1.8 L IS? (Ok, it'll be expensive).


And anything EF-S, if it's weathersealed, especially on the ultra-wide end like EF-s 11mm f/2.0 L, or 9-16 f/4.0 L. Anything above 16mm and there's sealed alternatives (although they're expensive, at least they exist).