I was hoping for some primes to compete with Canon's 400/500/600mm primes. A 300-600 would be interesting, but it likely won't be f/4 throughout; probably something like the Tamron instead as the weight would get too significant. Ah well; I can still hope for something in the future.
I'm sure they are coming, although I doubt they will deisng a 400mm f2.8.
Their 500mm f4.5 is a well regarded lens and one which seriously do with an update. An IS unit, opening up to f4 and the ability to take teleconverters without Af issues and weather sealed should do the trick.
The current Sigma big primes are over-heavy, over-priced (£4799 for the 500 f/4.5 and £5499 for the 800/5.6) under-perform in MTF and lack IS. To compete with Canon they will have to improve hugely and keep the price really down. When you want the ultimate, a £1000 here or there in £5000-8000 is not usually a deal breaker. I would like an ultra-light very sharp 400, 500 or 600 f/5.6 at a competitive price and leave the f/2.8 and f/4 to Canon.
5D IV, 5DS R, 7D II, EOS-M, Powershot G3 X, 400mm DO II, 300/2.8 II, 1.4xTC III, 2xTC III, EF 1.8 STM, EF 70-200/4 IS, EF 24-105, 15-85, 100-400 II, Sigma 10-20, Sigma 150-600mm C, EOS-M5 15-45, f/2 22, 11-22, Samyang 8mm f/2.8 fisheye