Recently, data miners poking around in the BIOS files of upcoming Gigabyte X570 motherboards have come across references to an "X590" chipset, which does not officially exist yet. In these BIOSes, many options are labeled as for "X570/590 chipsets." The developer for the Ryzen DRAM Calculator also hinted at an X590 chipset on Twitter, saying "AMD X590 and more possibilities coming soon" with an attached picture of a mostly blacked-out motherboard save for the corners; the Tweet has since been deleted.
If X590 exists, it could deliver more features oriented to complement AMD's highest-end Ryzen 9 3950X, which is essentially a low-end Threadripper replacement. X590 could have more PCIe lanes or higher quality motherboards than what already exists for X570 (say with a more aggressive clock speed by default), or just generally be more workstation focused. With 16 cores on the mainstream and the Zen 2 Threadripper CPUs not on the market yet, it might make sense for AMD to use the 3950X as a replacement for the 2950X and as a stopgap because the 3950X has been shown to do well against the 18-core Core i9 9980XE.
It's also entirely possible that X590 is actually a misnomer made by Gigabyte. If Gigabyte wasn't sure what the chipset would be called (remember the canceled Z490 chipset for AM4?), then it's entirely possible Gigabyte internally referred to this upcoming chipset as "X570/590." The X590 listing is never referred to alone, always with X570 right beside it, and technically it's not stated in the BIOS files at all, it has only been inferred with a listing of "590." And like Z490, it is possible that X590 was planned at one point, but also canceled.
Ultimately, however, much of the discussion on X590 is speculative because no official information about this supposed chipset exists, and even unofficial information is limited to some reference in some BIOS files and a single tweet. Considering how premium X570 already is, one has to wonder how X590 could possibly differentiate itself as a higher-end chipset.