Heya,
The 50mm was the most obvious, and they made it already. It filled a big gap where Canon only had poor offerings (50 F1.8 & 50 F1.4, lower end lenses, decent, but far from legendary) and then the expensive 50L which isn't even a superb lens, just a "character" lens. Sigma filled the gap with a technical lens that is simply excellent and not just a "character" lens with a price tag. It fits nicely between the cost of the lower end models from Canon and the Canon L which to me is more of a novelty lens than anything (sort of like the 50 F1.0L).
From there, they went with other things to compete with Canon. 35mm made a lot of sense. Canon has very good 35mm offerings, so they needed to get their competition in there. It's better than the 35L, but the 35 F2 IS is a big piece of competition at cost. Still it lands nicely between the two and fills a small gap.
The already dropped the 24 ART and it is better than the 24L, but that's no surprise. Sort of a niche lens compared to 35mm and 50mm which are far more commonly used in the general market most likely. A good release to compete with an already popular lens and focal length, without being too niche/specific.
They would be kind of silly to go for the 135 ART at this point. The 135L is too affordable and can be had often in the $700 range. The 135 ART would be better, no doubt, but it would have to be priced under that 135L price to compete, otherwise, the 135L reputation and availability would make the 135 ART not sell much other than to someone who wanted to pay more for a better technical lens, but really, it would be rather insignificant on the whole I think. They better save this one for later as they no doubt are fearing the release of an IS enabled model from Canon. If Sigma was able to release a 135 ART with OS, it would be a winner, even if priced higher than Canon's current 135L.
Sigma already has an 85 F1.4, and it's a good lens. If they release an 85 ART, they will completely discontinue and replace the current 85 F1.4 from Sigma. Then they compete with the very good and affordable 85 F1.8 from Canon, but provide a much better alternative to the ultra-costly 85L II. The 85 ART would be a good release. It gives people the faster aperture option that Canon is not providing except at the L II level (with a ridiculous cost associated with the F1.2 and size/weight slow AF). The 85 ART would fit nicely between the affordable 85 F1.8 and the ultracostly 85L II. But, it would pretty much wreck their current line of 85 F1.4 that they already have as it would totally replace it. If it had OS, it would be a top seller.
200 F1.8 from Sigma, or even 200 F2, or even 200 F2.8 ART from Sigma, as long as it had OS, would be a big seller. But it would likely cost nearly as much as the current Canons. They would however provide an alternative to a $4k lens. But this is a super niche lens. People wouldn't go out in droves after this. It would be costed higher than the average buyer would go to ($2k~3k easy start here). And it's focal length is a niche focal length in general for a fixed focal length lens. It's an extreme niche lens. Not a smart lens to drop, from a financial stand point, since the market they would intend to compete in is already extremely limited and small. It would be a nice lens though I'm sure.
So overall, I want to see the 85 ART.
Sigma would make their most impact to the competition by hitting the market with 35 ART, 50 ART and 85 ART to me. They're almost there as it is.
135 ART would be the weakest release in terms of market, in my opinion.
Very best,