Back in June, Frontier Developments announced that the Thargoids, Elite's infamous alien bad guys, would return to Elite: Dangerous in the 2.4 update, appropriately entitled The Return. The studio said at the time that it would be ready for release in the third quarter of this year, and so it will be, as the launch date has now been nailed down to September 26.
"On launch day you’ll be interacting with the most mysterious and deadly things you’ve encountered in the Elite Dangerous galaxy – the Thargoids," community manager Edward Lewis said in the Elite Dangerous forums. "Elite Dangerous 2.4 The Return will then continue to roll out with interactive narrative-based content emerging over the coming months."
There's no gameplay in the cinematic trailer that was released alongside the announcement, although that shouldn't be much of a deal-breaker for a game that's been out for almost three years, but it makes the situation clear—not that I want to spoil it for anyone, but this is definitely not just another bug hunt.
Frontier didn't reveal anything more about the 2.4 update in today's announcement beyond the release date (and that the Thargoids are seriously bad news), but that's not terribly surprising, given what Lewis said about it when the return of the Thargoids was first confirmed. "As a story-centric update, showing too much ahead of time would spoil the surprise and make for a lesser experience for the community," he wrote. "We want The Return to surprise and shock you as much as possible."
Given the timing, I'd say it's a fair bet that Frontier will have more to reveal about the Thargoids at Frontier Expo 2017, which will take place on October 7 in London.