For the sake of this thread, I'll define a "nerf" as "a reduction in AP generation."
Nerfs aren't going to do it. Nerfs mean that spammers just need to spam a little harder. Fixes are what really need to be put into play. Things that are truly oversights that give an unfair advantage of one character over another.
Kratos: As far as I can tell, he's the only character in the game with an attack that "homes in" on an enemy (↔☐). Having a homing attack in a fighting game is like bringing a gun to a fist fight. Fighting games are about precision and timing, not grab-the-closest-dude-and-toss-him. (I have heard that Nariko has a move that somewhat homes, but if this is true, instances of spamming it are far more isolated than with Kratos.)
Raiden: I didn't realize how big of an issue his kill confirm is until I fought a Raiden in 1v1. It's impossible to dodge his Level 1 after being grabbed. With Raiden, you know you're dead 2 whole seconds before he even activates the Super. All that really needs to be done is to give the opponent the ability to roll out of the way before standing up. In addition, this video shows an infinite combo of his that desperately needs to be fixed.
Nathan Drake: While that barrel really frustrates me, I won't nitpick about it. He has an infinite as well (seen here), and while it doesn't always last forever, it wastes a ton of time and is obviously unintentional.
Toro: He has a combo that keeps him airborne for as long as you want, which some players take advantage of in 2v2 or 1v1 matches (get one kill, then float above the stage for the rest of the match). This is a VERY isolated issue, but nonetheless needs to be fixed.
For the most part, kill confirms/combos do not need to be fixed, as many of them are totally dodgable. Most of the kill confirm videos I have seen are against braindead CPUs in practice mode and are thus wholly unconvincing. Also, I don't think Projectile Campers should be fixed, either. Yes, Radec/Evil Cole campers can be annoying, but they are completely managable.