It's hard to say. If there was a 100-500, the slowest it would be at 500mm would be f5.6 and I'd expect a 100-500/4-5.6L IS lens to cost well over $2000. The front element of a 500/5.6 lens has to be at least 90mm in diameter, which is pretty big. Remember the 500/4L IS is only one stop faster, is a far simpler optical design and sells for over $5000. The Nikon 200-400/4 lens also sells for ober $5000.

So I don't really expect to see a 100-500 from Canon. The current 100-400 is relative small and light and affordable. A 500mm version would be larger, heavier and a good deal more expensive.

All that being said, Canon have surprised me before, and probably will again!

Unless you can afford the 500/4, the 100-400/4-5.6L and 400/5.6L would seem to be your only alternatives if you stick with Canon and want a significantly longer lens. Since you already have a zoom for 70-200 (and up to 280 with the 1.4x), maybe the 400/5.6L would make the most sense. Very sharp lens with fast AF. It doesn't have IS, so it may require some sort of support (beanbag, monopod, tripod) to get the best out of it.

You can get a Sigma 50-500, but it's f6.3 at the long end and the quality isn't as high as the Canon lenses.