I'm tempted to recommend an UAW, a macro, or another fast lens. Something that gives you more creative ability over what you already have. This of course is going to come down to budget. "Cheap" creative fun would be the 17-40L, or the 85mm f/1.8 <--- also good for sports, but might be short for outdoors. The either 100mm macro would be great for creavitity and can be used for sports, but the AF might be lacking. On the longer end, for those across the field shots or wildlife/birds/planes/moon, you could be looking at the Sgima 50-500mm OS, 150-500mm OS, Canon 100-400mm IS, or the new Tamron 150-600mm VC. The sample pictures of the Tamron have almost pushed me far enough to buy it, but I am waiting for more reviews, results, sample pics before I buy. I'd like to make sure there are no reported bugs that may need fixing.
5D4 or 6D2..... Waiting to find out which I buy | 8-15L |24-70mm f/4L IS | 24L II | 40mm pancake | 100L IS | 70-200mm f/2.8L IS mk2 | 400mm f/4 DO IS