I hope Sigma or Canon will make a new lens category at some point. Such as an f/3.5 300, an f/5 400, or a zoom with limited range but fast aperture, such as a 250-450 f/5, or 200-350 f/4.5...all of which with stabilization. It's kind of annoying that telephoto lenses go from the $1500 range, to the $3500-$13,000 range...with nothing in between.
I am thinking Tamron might step up here. They now have VC and USD capable enough for this area. A good way to differentiate themselves from Sigma/Canon would be to provide some new combinations of fast primes, like 250mm f/2.5, 350mm f/3.5, and 450mm f/4.5... They would probably end up in the space 2-5k$ (keeping my fingers crossed)