When you shoot wide angle now, adjusting for the correct field of view are you at 24, 28, or 35mm most of the time. What mid zoom range do you really need, Is it 24-70 or 24-120. I would not consider the 24-120 slow. Most 24mm primes need to be stopped down a little any way and you will be shooting at f4 of f5.6 probably more than you will shoot at f2.8.
If you really need a fast wide prime then the 28mm f1.8 or the way more expensive 24mm f1.4 would be a good choice. Sigma just came out with a 35mm f1.4 that might be very good, although probably not wide enough for your needs. You have a very nice 50mm now, so another option might be the Nikon 16-35mm f4. Its great lens and exceptional in the 20-30mm range, with the 50mm f1.4 you might not really need a true mid range zoom.