If I had to pick one, buffing the underpowered characters would be it.
Jak, Spike, Sir Dan, they're all really fun for me to play as, but at least Jak and Sir Dan are underpowered (while Spike may be mid-tier). I don't want to simply resort to picking a mid-high tier character just to be able to compete. I want to be able to compete with the characters I actually have fun with.
I also like Sweet Tooth, and while his moveset, AP gain, and level 1 (with a hit confirm) are great, his level 1 really doesn't work against air camping. His level 2 really needs to be buffed to counter air campers, and that huge gap you have to sit in building up to your level 3 due to his level 2 being terrible is agony. Otherwise, yep, he's great.
Every patch for PlayStation All-Stars seems to fix one thing and break another. It's like fixing the windshield of a car, and then the tires pop off.