I was confused on one part with Scarlet Witch

It wasn't just fears. She showed them the darkest parts of their minds.

And even if it was just fears, not everyone has fears of something that could happen. Some people's greatest fears are things that have happened. That a particular memory is when they were the most frightened.

You are sin.

It could have been a fear of becoming that person again, or of still having that part of her that used to kill emotionlessly inside her. A wondering if she can ever truly be free of that part of herself that was created in the red room.

Natasha's fear is that she has no purpose other than to end lives. That is what she and the other girls were trained for, any chance to create new life was removed during her graduation ceremony. The memories were reinforcing that fear.

Formerly l5rfox.
And there was one world with nothing but shrimp. I tired of it quickly.

Yeah as others have suggested it wasn't as simple as "fears" it just the darkest part of their minds. Self-doubt, regret, guilt etc.

Tony's vision, which we see the most of, has all of these really. Mostly fear of not being able to protect the world but heavy on self-doubt as well. Regret and guilt too in that "you could have saved us" which I took to partial be about how people did die in NY and if Tony had been smarter/better he could have stopped Loki before the portal was opened. It was his power source that let it all happen and because it was his power source, that's why it happened where it did. Any other similar power source (nuclear plant) would be in a more remote are with less population. So the very fact that he built his arc reactor in downtown Manhattan put all those people at risk.

I'm pretty sure that after the attack on NY, Tony took precautions to make sure nobody could ever use his tech in that way again. Just like way back in IM1 he went and found/destroyed all the weapons that fell into the wrong hands.

Reading my signature constitutes admission that I am correct. (Too late)