If you saw Avengers: Infinity War, you'll realise that Thanos is one mean dude and pretty much defeated the Earth's Mightiest Heroes single-handedly - or at least with a click of his fingers.
Presumably, the plot for the as-yet-unnamed Avengers 4 will revolve around the remaining Avengers attempting to stop the Mad Titan and bring those that were slain back to life.
While that's bound to happen in some capacity, it has also been hinted that there might be other, more powerful villains joining the fray in the next movie.
A tie-in but not necessarily canon Marvel novel named Thanos: Titan Consumed, written by Barry Lyga, is intended to serve as an origin story for Thanos and how he became obsessed with the Infinity Stones.
Screenrant report that Lyga did work closely with Marvel Studios when writing the book and made sure that it was completed well in advance of the release of Infinity War so there could be something to this, even if it isn't 'canon'
Towards the end of the book, Thanos encounters a character known as 'the Lorespeaker', who informs him about a powerful race of aliens known as the Celestials and an even more powerful group that they fear.
The passage accordingly goes as follows:
'They are hidden, Thanos,' the Lorespeaker said, as though badgering a child, 'because they are too powerful. And because the Celestials and the others, the ones whom the Celestials fear, keep close watch on the Stones from afar.'
In comics, the Celestials aren't good guys or bad guys - they are just neutral. But they might make for villains if tested by the Avengers in their quest for justice.
It's worth noting the Ego the Living Planet from Guardians of the Galaxy Vol. 2 was considered to a Celestial, while other members of the race have popped up in other Marvel movies but haven't been directly referenced.
This is what they look like in the pages of Marvel comics.
As for 'whom the Celestials fear', your guess is as good as ours, but the villain Annihilus has been hinted as being the next major villain for the franchise going forward, so it could be him.
Of course, this could all be absolute nonsense and hearsay spread by Marvel fans on the internet, who are tired about not having a synopsis or a trailer for a movie that is supposed to be released in less than six months!
Still, it's fun to speculate which is just what folks on social media love to do.