Which characters need buffs?

Imo most of the vanguards do.

Magneto: His ult needs to get rid of the overcharge. It's too easy to get no value from his ult.

Venom: Lowest win rate among tanks at all levels. His base ult damage should be 100 at least instead of the current 50.

Groot: Ult damage could use an increase. It's only 150 right now.

Captain America: Let the man put his shield up when he wants. Hit ult literally does 0 damage. Let Cap play defensive.

Hulk: His ult's smash and grab should do 275 damage at least. Currently it only does 200 so it won't kill any full health character. It's also a long animation that takes up most of Hulk's ult time and can be easily countered by healers. This makes doing the grab and smash animation while ulting not worthwhile. Their should be greater reward for getting it off.

Thor: Needs some quality of life adjustment with aiming his ult. Slight damage boost would be nice.

Jeff has the lowest win rate among supports and second lowest in the game. Not sure how he can be improved it seems to be related to his lack of defensive ult compared to other healers. Maybe it's fine he stays a niche character.