I want a lens that does not yet exist. I would like a 24-135 f1.4-f2.8 L with IS. Preferably less than 800 grams; black, not white; and using a 77mm filter.
That would be my perfect walkabout lens (the the perfect kit lens). The 24-105L is really nice, but I wish it were faster. I've finally come to appreciate the advantage of the faster lens with the 5D3 AF system.
I also like the idea of a modest-range walkaround lens. Longer reach than 70mm, but not going crazy like the 28-300mm to reduce distortion and keep size/weight reasonable. However, I think this one sabotages its chances in two key areas:
1) I'd be shocked to see a zoom with that kind of range open up to f1.4. Even f2.0 would probably be fantasy. Consider the 28-300mm which has f3.5-5.6, or the 28-135mm which also has f3.5-5.6. I think we'd be lucky to get f2.8-3.5 over a 24-135mm range. Throw in IS and there goes your weight criteria.
2) If they sold a great-quality lens in that range, we'd have no need to buy any other bright primes or even the 24-70mm f2.8 ii! The kings of marketing and product-differentiation would never allow it. That reasoning explains the good-but-not-great IQ of the 24-105mm.
But as long as we're dreaming... count me in for one also!