I own the Sigma 14mm f/1.8 for astro. It's great! but it's a DSLR lens, and Sigma makes it for the Nikon F mount, which means they had to design their optics around a very small opening with a big flange distance. That's a big deal for fast, super-wide lenses, which benefit the most from the wide openings and short flange distances of modern mirrorless mounts. Canon could certainly make a 14mm lens one stop faster for their R mount. No doubt it would be huge, but so is the Sigma, and it's always on a tripod, anyway. BTW, the vignetting isn't ever a real-world problem for me... We've been testing the Canon 24-240 RF which has serious vignetting and distortion at 24mm, but the camera and Lightroom both automatically correct for it in post, which means you never see any problem with your images. I am fine with that.
Well, if Canon could make a EF 200mm f1.8L is no reason they couldn't make an RF 135mm f1.4L and an RF 200mm f1.4L. Hard to say if they would put IS in them or not. However, I wouldn't want to even guess how much an RF 200mm f1.4L would sell for.
Assuming that the "consumer grade" RF lenses are definitely coming, I'd like to see some movement from Canon on "entry level" RF L-series lenses. Against its predecessors, the 24-105 f/4.0L is freaking spectacular, and it's not an awful price point. A 100-400 f/4.0 or 4.5L and a 10-24 f/4.0L are well within reach, and would make a pretty compelling "Entry Level Trinity" of f/4.0 lenses. The lens I use the most day to day is my 16-35 f/2.8L, and the lens I rent the most is the 100-400 f/4.5-5.6L II. At f/7.1, I'm a bit reserved about the slow F stop on the top end of the announced 100-500. I would love to see them complete a "trinity" lineup that would drive sales into the R line.
I am all for the 14-35 f4 that is filterable. I also vote for the fast ultra-wide astro lens (14 mm and at least f 1.8). Beyond that I would tend to the opposite end of the focal length. Relatively fast, relatively sharp 100-500 or 200-600 and a light 600 mm f4 (DO or otherwise). I have the 100-400 and 600 f4 vII and can adapt both of these but would replace them with R mounts if they were lighter, faster, and as sharp.