What does "natural light photography" entail? Meaning you don't want to learn any kind of supplementary lighting? There's really no good reason to use only natural light, other than the fact that you have literally no desire to learn flash.
Not giving a price range for a lens you're looking for is also kind of tough. I'm a fan of fast primes, so I'd suggest them over alternatives.
What I carry around on my 5D, and 5D2 pretty much all the time is a Sigma 85mm f/1.4, I also use the Canon 50mm f/1.4. The 85mm stays on my camera 85% of the time. I am severely lusting after the Canon 135 f/2L which performs marvelously on a full frame body, and is outstanding for portraiture.
Other options that you may find more versatile (and more expensive) than primes are the 24-70 f/2.8L, but over that I'd recommend the 70-200 f/2.8L. Having the ablility to go from 70-200mm at a constant f/2.8 aperture is very valuable, and very versatile. It's also worth quite a hunk of money, unless you buy the MK I version used which is what I'd suggest.
Thank you. I do Flash photography as well, but I love outside lighting. And I also have the 50mm 1.4. I love that lens but I want a zoom lens. One for weddings and I can go up to $1200 for a good lens..... ???