I too have kept the 50 and the 70-300. The way I see it, you could either go the 16-35 route, to round-up things and truly expand wide angle, or get the 28mm and 85mm f1.8G, for a great trio of primes. I'm guessing that's the path I'll take.
http://egozarolho.blogspot.com 1. Good content, good aesthetics and good tecnique. On that order. 2. Light is more important than glass and pixels. 3. In the digital photography process, software is as important as gear.