The 24-105L is pretty lonely with that wide range, at least from Canon. There's also the 28-300L, but other than that, it's just the 24-70L. I too would find its range a bit too short for my main fancies. But there's a reason for this 24-105 loneliness: it's just great, there's no need for any others.
- Getting primes for low-light situations is a good idea.
- The 24-105L is great, even though it's f/4, and it will surely outresolve 22Mpx.
- The increased light performance from the 5D Mk3 means you can use these lighter, wider-range, smaller aperture lenses in the same situations. You're buying the possibility to take a step backwards lens-wise on light performance, while still being in decent condition due to several steps forwards from the camera body. And on several other aspects, the lens is a step forward, too. You get slightly shallower DOF wide open, increased sharpness, much better build quality and weather sealing, bigger zoom range, and less weight (compared to an f/2.8 lens, that is)...
- Consider the Canon EF 50mm f/1.4 (pinched to f/2) and the Canon EF 85mm f/1.8 (wide-open) as some decent cheap&sharp primes.