What does the vision of OG 6 Avengers in Age of Ultron tell us? Was it a vision of their future?

Undoubtedly, it was a glimpse into the future.

Age of Ultron opens with Tony Stark seeing a picture of the World being overtaken by Chitauri and a mass of fallen Avengers.

Tony wasn't the only one who had visions, the premonition of what was to come was also experienced by other 4 OG avengers, these visions clearly connected with what happed in Infinity War and Endgame. For Tony it was nightmare, he was shown that what could happen if he did not volunteer.

The very first glimpse we get is of Hulk, who no one thought could be defeated, despite the fact that he was completely decimated. However, he was visibly breathing, indicating that he wasn't dead. This hinted at what would happen in Infinity War, specifically, Hulk losing the fight to Thanos.

Thor is lying down with his eyes closed, implying that he is unconscious rather than dead. You could also argue that Mjolnir is just out of reach for Thor, which could be a hint as to how he lost Mjolnir in Thor: Ragnarok, although that one is a bit far fetched in compared to the others. This vision also links to the destruction of Thor in the first scene of Infinity War.

Hawkeye, unlike the other Avengers, does not appear to be physically beaten; instead, he is shown sitting up, shoulders drooping, and head down. Now, it appears to me that Hawkeye is sitting there mentally damaged rather than physically broken. Implying the emotional breakdown Clint goes through after his family is snapped and once more after he losses Natasha.

The Black Widow is depicted as lying motionless with her eyes wide open, implying death. This connects to the death of Natasha in Endgame. 

Perhaps the most iconic of all, the broken shield. Captain America was also unconscious with his eyes closed, but he awoke to speak with Tony. "You... could have... saved... us," Steve tells him as he dies. As we all know, Thanos shattered Captain America's shield during Endgame in a manner eerily identical to the destruction Tony saw in his vision.

Another thing Tony sees in his vision is Chitauri returning in waves to Earth to launch a huge attack. And, of course, we know what happened during Endgame's climactic showdown. And, looking back on Avengers: Endgame, I believe that was a purposeful inclusion to the scene as a foreshadowing of what was to come.

The only problem was that it never showed Stark how things would turn out if he tried to save everyone. Stark was so afraid to lose after this vision that it compelled him to snap in Endgame, in fear that if he does nothing all can die.

If you recall, Wanda Maximoff, the Scarlet Witch, caused Tony's vision in the first part of this film while she was working with Hydra. She also caused similar vision for Steve Rogers, Natasha Romanoff, Thor, and Bruce Banner (thus setting the Hulk loose on Johannesburg).

Steve Rogers imagines himself dancing with his long lost love, Peggy Carter. Peggy informs Steve that the war is finished and that they may return home, but he is then left alone in the dancing room, reminding him that all of these people have died (or dying).

Fans believed Scarlet Witch was showing Steve the life he may have had at the time. Steve does however, reach that life in Avengers: Endgame. He travels back in time and lives a quiet, happy life with Peggy. Steve and Peggy dance cheek-to-cheek in their house at the end of the Avengers: Endgame movie, just as they did in Captain America's vision from Age of Ultron.

Natasha doesn’t see a version of the future, but rather her past, she sees her past in the red room, reminding her that she has no place in the world, and no future, no family, that she isn’t part of anything, thus making her doubt herself, and her place among the Avengers. 

In a way, though, this is also Black Widow’s future. This scene indicates that Natasha will be going back red room and in the Black Widow movie, she indeed does go back to the red room and by the end of the movie she overcomes her self doubt and understands that she has two family's. The Avengers and her red room family, and that both the families need her.

Thor watches his fellow Asgardians die, presumably by his own hand, and Heimdall tells him that "They see you leading us to Hel" and that the God of Thunder "is a destroyer". Thor was a man who craved battle and power, and he probably doesn't want to be that man again. He is afraid that his poor leadership would lead to his people's demise.

In some ways, this is true: in Thor: Ragnarok, Thor battles Hela, the Goddess of Death. By reawakening Surtur on himself, he practically destroys Asgard.

Then, in Avengers: Infinity War, he loses to Thanos, who boards Thor's escape ship and murders Heimdall and many others onboard.

and then again at the end when he fails to kill Thanos.

In the Endgame we see Thor blaming himself for failing to prevent so many deaths in Endgame, as though he slaughtered the Asgardians himself.

Bruce Banner's worst nightmare is to kill innocent people, and he is terrified of losing control in a crowded place. This message is emphasized several times throughout the films.

We have no idea what he sees in his vision; all we know is that it sends the Hulk on a frenzy. As a result, it most likely played on his fear or fury.

Am not quite sure what Hulks vision indicates but maybe it shows us how Hulk overcomes his fear of turning green by creating a understanding between Hulk and Banner in the form of a shared body and consciousness, resulting in Professor Hulk.

Hawkeye was the only Avenger who evaded Wanda's mind control, he was fast enough to stop Wanda from showing him any visions, so we didn't get to see Clint's fear or nightmare.