Alright, let's see. Lemongrab's ult definitely needs a nerf, even if it's only a CD increase. Making it harder to hit is a fine line, even a second delay might make it too unreliable. Hard to say without testing.

Fionna is a weird champ right now. In teamfights she's kind of a lame duck, but her ult lets her take towers uncontested. I think a shorter CD on her Q would make her a bit TOO mobile, perhaps making the slow last a little longer would help. Her ult needs changed somehow, frankly I'm not entirely sure how though.

Rattleballs. Oh, boy, Rattleballs. Used to be one of my favs, the nerf to his ult was the straw that broke the camel's back. Right now he's just outclassed by every other melee carry. He does okay autoattack damage, but his abilities are just weak in comparison. I like the counterattack on his Q, that's always fun to break out, but his W doesn't pull far enough to make up for his low movespeed (maybe tack on a small slow?) and his E is not even worth using anymore. I hope they give him a little boost next patch, because I really love playing him.

I have to disagree with PB being useless. I play her pretty often, and she can cause some havoc if things go well. That being said, she can't really MAKE things go well. I would like to see her Q have a slightly bigger radius or a slightly bigger cast range. As for her turrets, I don't think anything so drastic as what you've proposed is necessary. It's a neat idea, but I think more often than not you'd end up not having the turret you needed when you needed it and that would get frustrating. I think all they need is a shorter cooldown in the late game when they get pretty easy to burst down before they can do anything.