The 24-35/2.8 range is already covered by two lenses. I really can't see adding a third one unless it brings with it something extra. For example, what about a 24-35 f/2.0? Or even faster?
If you need ultimate quality, just get a set of primes.
Three primes to be exact...
We're talking about 24-28-35. Existing line-up is 16-35/2.8, 24-70/2.8. Both of them have weaker performance on the upper or lower ends and are more bulky than compact. I don't mind seeing Canon launch a 8-500mm for the sake of portability, but that's not gonna make me buy it.
Forget about brightness for a while. The way things work is you launch a made-to-measure core model, then you expand on it for the all-rounders. Core model is 24-35. You may launch 8-35 or 24-100 if you want to but the centrepiece is 24-35.
The only marketing problem is that a real good 24-35/L may threaten sales of your primes. On the other hand your sales of 24-35 rise. So, in the end there's profit anyhow.