Despite finally fulfilling his destiny to become the ruler of Asgard, Thor has been having terrible nightmares recently in which he's slain by Thanos. Ever since the Black Winter showed him how he was going to die, he's been terrified, expecting to be crushed to death by Thanos wielding an Infinity Stone-powered Mjolnir and a seemingly Black Winter inspired all-black arm.
Thor has been having these visions throughout the current run of Thor - by Donny Cates, Nic Klein, and Matthew Wilson - but Marvel has shown no sign of altering the Infinity Stones' status quo, with the omnipotent artifacts seemingly bonded to their new hosts for the long haul. It seemed unlikely Marvel would change its new approach to the Stones just to empower Thanos yet again, but because of the upcoming Timeless one-shot, a future where Thor's vision comes true is finally starting to seem plausible.
It has been up in the air as to how Thanos was going to get control of the Infinity Stones again, since Earth-616's Stones are currently bonded with individuals, and Marvel's Infinite Destinies event just made it clear the comics are invested in exploring their stories. However, Marvel's recent marketing for the Timeless one-shot has made it clear that Kang will find himself dealing with alternate timelines. Marvel Comics recently embraced the MCU's definition of variant timelines, making it seem likely that the Thanos closing in on Thor is a Mad Titan from another reality - one Kang will visit in Timeless. Having the Infinity-Mjolnir carrying Thanos be a Multiversal variant from a rogue timeline is the perfect way to introduce this terrifyingly powerful version of the Mad Titan without reversing recent developments to make it happen.
The Timeless teaser asks, "What weapon will Thanos wield to threaten the future of the Marvel Universe?" This image of Thanos with the Infinity-Mjolnir raises even more questions, connected to the lore of Mjolnir as well as the Infinity Stones. Mjolnir is meant to only be able to be lifted by those who are worthy, and was recently stolen from the Avengers' custody. The hammer's enchantment has been haywire recently, allowing anyone to lift it, so it's possible the Thanos who will slay Thor is already in possession of this deadly weapon.
The Timeless event has been marketed as a chance for Kang the Conqueror to be a "good guy" and save the 616-timeline from a dangerous rogue timeline. Other teasers for the upcoming event include a brooding Doctor Doom, the return of Eternity's son Entropy, and the introduction of a new character, Anatoly Petrov. With all these possible variants and potential futures, Kang, the Avengers, and other Marvel heroes will certainly have their hands full, with King Thor almost certainly taking on the Infinity-Mjolnir carrying Thanos. Coming December, Timeless seems set to finally reveal what Thor's vision of his death at Thanos' hands actually means.