I am a big fan of Sigma lenses. In fact, I think most of my lenses are Sigma lenses. I really like the Art-series prime lenses. So I'll be glad to see them come to RF.
I don't have any inside information, of course. But while we don't know which lenses will come to RF first, we do have a pretty good idea of how they'll be in a number of ways.
All Sigma optical formulas are designed to be usable across mounts, so the lenses we get on RF will be identical optically to those already out for E- and L-mount cameras.
The 50mm f/1.4 DN (which was released this year) is probably not much more than a side-grade for RF users from the EF predecessor. Make no mistakes: it'll be a solid lens. But the EF Sigma is already very good.
The DN improves corner sharpness, but at the expense of introducing significantly more geometrical distortion. The DN improves purple fringing a bit, but chromatic aberration levels are fairly similar overall.
The focus motor is quieter, but some may prefer the manual coupling of the EF lens to the focus-by-wire. The DN will probably bring a moderate improvement in AF speed. I'll comment on AF in general below, though.
Anyone expecting a big difference in size and weight may be disappointed. The lens does get smaller and lighter, but it's hardly in a different league. (The older Sigma 50 is already one of the small...er EF Sigma Art lenses, to be fair!)
While the CR article says that we're not sure about Art or Contemporary, if it's f/1.4, then it'll be Art; the Contemporary Sigma 50mm is f/2. That lens is quite different from its bigger f/1.4 brother.
If there's a lens like an APS-C 17-50, I highly suspect it would be their 18-50mm f/2.8 (which doesn't have optical image stabilization, by the way). I highly doubt that they'd have a new optical design in the works for something so similar to a mirrorless lens they already make.
Now, putting aside the specific lenses that might come first, I'll say that I don't think the jump from EF to RF Sigma lenses will make that huge a difference (putting aside the optical differences, which we can already assess from their performance on other mounts). EF lenses work exceptionally well on RF cameras — and this applies to Sigma lenses, not just Canon ones. I've used Sigma EF lenses on a Sony camera via the MC-11; the same lenses were considerably better at focusing on the R5. The Sony-native versions of Sigma lenses with DSLR optical formulas clearly exist for a reason. I don't think we'll ever see HSM Sigma lens formulas ported to RF like they have been to E. There just wouldn't be a real advantage to it. (I'll mention that Sigma, unlike Tamron, has not been discontinuing their EF-mount lineup. Not only that, they still actively promote using them on RF cameras!)
I would personally be most interested in new lenses like the 35mm f/1.2 or 14mm f/1.4. I'm pretty sure we will see these eventually. Although it's not just a matter of whether or not Canon will let it happen; I think the 50mm f/1.4 is probably a top seller, and those other lenses are more niche. Sigma would probably want to open with a mass-appeal lens. Eventually the entire line of DN lenses will come over, though, surely.
Anyway, this is all a little while away. In the meantime, I'll still be using my EF Sigma lenses. (Well, even after they launch RF lenses, I'll still be using my existing Sigmas...)
