In terms of optical performance, Sigma are smashing it out. The 35 and 50 Art lenses are amazing but their 24-105 zoom not so (or at least not much difference to Canon's.)
In doing a 14-24 and 24 lens, Sigma clearly see a golden opportunity to deliver new lenses into a market segment that is not very well served (wide and ultra wide angle).
If they can deliver a 24-70/f2, that will raise a few eyebrows but how heavy will it be, etc? And for a lens like that, AF problems will not be acceptable because that market is a lot about indoor shooting at weddings, etc, where if you don't nail it with the first shot then you haven't gotten the shot. So they'll need to be on top of their game for that one.
But that makes me wonder, what other parts of Canon's lens lineup is weak? Where would you go ir you were Sigma and looking to capture a waiting market?
85mm Art? Is there anyone with a Canon 85/1.2 or 85/1.8 that is unsatisfied with their lens?
100mm? Again, do you read about anyone bitching that their 100 is no good? Or 135? Or 200?
When it comes to zooms, it's a bit more interesting.
They have a 14-24/4.5-5.6 that will likely be replaced by the 14-24/4.
They have a 24-70/2.8 now but rumors are that a 24-70/2 will replace it.
The 150-500 is kind of in a league of its own.
But then there's a 24-70, 120-300, 120-400 and three 70-300s. None of which are particularly good (especially the 70-300s.) In that space, Canon has a 100-400 that is well north of $2000, a 70-300 consumer grade lens that isn't worth owning (even Tamron's is better) and a 70-300L somewhere between 1200 and 1500 (depending on rebates.)
Could Sigma do a 70-300 Art at or just under $1000?
Or maybe in the $700-$800 space that provides enough value for money over Tamron's lens?
What about a new version of the 120-400? With Canon due to replace that soon, waiting to see how Canon position that in terms or price vs optical performance is sensible.
I can't see them replacing the 120-300 unless they introduce another medium zoom that os X-120.
There's a hole in the market for a good performing telephoto zoom in the 70 (or 100) to 300 for full frame that delivers good optical performance across the frame and is sub-$1000 (although I suspect that it needs to be more a sub-$800 lens to lure away buyers from Canon's 70-300L.)
If you were Sigma, what part of Canon's (or Nikon's) lens lineup is weak and easily attacked?