The following observations are from playing each character for multiple hours in PvP. These are changes to each character that “I” personally think would help. Also forgive my spelling of character names, I am on my phone and do not have a list in front of me.
Oscar Mike - Decrease magazine size by 20% or decrease base damage of primary attack by 10%. Decrease burning damage per second of fire grenade by 15%.
Marquis - Decrease base damage while scoped by 15%, or decrease range of effectiveness by 20%. Increase health by 200 or add 100 to shield. *damage range reduction needs to be implemented so he is not able to control a team of 5, but rather 1 person as a sniper should.
Phoebe - Decrease CC time against players by 20%. Decrease attack speed by 15%.
Galilea - Decrease vase damage by an additional 15% from previous nerf or decrease healing ability when transformed by 25%.
Ambra - Increase sunspot health by 25% or change sunspot to only attack to a certain number of enemies at once i.e. 3 or 5.
Montana - Either decrease base damage by 15% or decrease health by 15%.
Galt - Fix shotgun going through enemy without causing damage. Increase movement speed by 15%. Increase distance traps can be placed.
Kelvin - Decrease health by 15% or amount of shield gained from skills by 15%.
These are just things Ive noticed and have had personal experience with while playing these characters. I have personally gone 20-0 multiple times with some of these characters. Not because I am any better than anyone else. But because I see the problems and exploit them. Please feel free to add any characters I do not have experience with. Thanks!