Though it really shouldn't come as a big surprise to anybody who's been keeping up with the MCU, The Guardians of the Galaxy will, in some way, be involved with the fourth Thor film. At least according to a comment that came out of the mouth (intentionally or otherwise) from the voice of Groot, Vin Diesel.
"I am waiting and excited for my friend James Gunn to knock this next one out of the park," said Diesel, in reference to the third Guardians film, while being interviewed to promote Bloodshot. "He took The Suicide Squad so he's about to embark on it. Thor will also, the director talked to me about Thor will incorporate some of the Guardians of the Galaxy."
That director he was speaking of was Taika Waititi, who previously helmed 2017's Thor: Ragnarok.
Diesel went on to point out that this was previously unknown news, as he said, "That'll be very interesting, nobody knows, maybe I shouldn't have said anything."
When last we saw this group of heroes, the Guardians were leaving Earth with Thor alongside them. Comic Book reports that the original idea was to have Guardians of the Galaxy Vol. 3 come out before the fourth Thor film, which would've featured the God of Thunder in their movies instead of the other way around. As Diesel alludes to in his quote, the script was flipped on the releases, which is why the Thor line of films is now set to incorporate Peter Quill and Co. At least, allegedly.
Whether or not they'll have a big role, or it's just something to explain what happened in the immediate aftermath of Endgame, is unclear for now.
Production on that film starts in August 2020, with a scheduled release date of Nov. 5, 2021. As for Guardians of the Galaxy Vol. 3? Well, that release is yet to be determined. But production for that starts next February.