Unfortunately, I don't see Sigma producing any dedicated designs for any FF mirrorless mount yet (they have a few inexpensive dedicated lenses for APS-C E-mount and Micro 43. Sigma L-mount lenses are DSLR lenses with built-in converters, just like the FE lenses. The E/Micro 43 lenses are likely to show up in the next batch of L lenses, since they're relevant to APS-C L-mount cameras - but I haven't seen any indication of a single genuinely new lens from Sigma for L or any other mirrorless mount in years.
If Sigma (or someone else) did decide to do a new full-frame mirrorless lens, they'd probably have to make it fit a mount of 20mm depth and 47mm diameter. Any design that needs a wider mount than 47mm loses the majority of the current market because it can't fit Sony FE. A lens that has to have a shorter back distance than 20mm would lose Canon RF and L mount.
The body manufacturers (especially Nikon) have some advantages over third-party lens makers, because they only have to fit their own mount, rather than the combination of FE's narrower throat and L and RF's longer flange distance. Nikon can design right to the edges of 16mm depth and 55mm width, and they're producing some very sharp, compact lenses. The L-mount makers (51.6mm) and especially Canon (54mm) can use broader diameters than third parties trying to fit all four mounts. Sony is constrained by mount diameter, but benefits from a slightly shorter flange distance than L or RF.
Another consideration for a third-party maker is whether to try and pick up one (or both!!!) of the Fujifilm mounts, despite their different sensor sizes. X-mount is right in the middle of the others for flange distance. It's notably narrower (44mm), but it might be possible to use a narrow bayonet that would vignette on full-frame, but covers APS-C just fine. For certain lenses, the focal length would make sense on either APS-C or FF. A fast 50mm would be a nice portrait lens on X-mount where Fuji's only 50mm is f2 (there's also a 56mm f1.2, which is large enough that a FF lens might not be too much bulkier - or it might be so good that the extra bulk was worth it to many photographers), and a very fast 35mm would be a fast normal lens (an average-speed 35mm would be huge in comparison to the very compact Fujinons). If the third-party lens were either a longish telephoto or a macro lens, the crop becomes an advantage - and Fuji is somewhat short on both.
GF mount, of course, requires a lens that has extra coverage on FF - but many longer lenses do, either unmodified or with alterations only to baffling, keeping the optics the same. By the time you reach a fast 300mm lens, it's hard to design one that doesn't cover medium format - I read somewhere that many of them would actually cover 4x5" if you removed the baffles. I wouldn't be surprised if most FF designs over 100 or 150mm would cover GF with few if any alterations. The longer flange distance of GF mount shouldn't affect the longer lenses that are most likely to cover the bigger sensor.