7D landscape lens

I have another lens questions. I have a 450D with 18-55 and 55-250 lens which is great for family shots and I've been using it for landscapes.

I have a 7D with the 100-400mm lens for wildlife. I need a lens for my 7D for landscapes and waterfalls. I've been using my kit lenes from my 450D, but wondered if I couldn't be getting better shots with my 7D with an upgrade. I don't have an unlimited budget, but any suggestions on a nice lens to add? I've read a lot on the site, but can't find an answer to my specific question. I almost never take photos of people with it, so it's usually just landscapes ,waterfalls and wildlife.
Any advice would be great. Thank you