What would be your suggestions specifically for Sugar, Law, Bonney and Perona?
Considering that the solution can't be too niche and still allows Oda to play all the scenarios we've seen so far?
Well I can see that this conversation has raged on since you asked this, but I'll give my opinion anyway.
Sugar - Sugar can turn people she touches into toys. Aside from that she's just a kid. I say debatably she might not need any nerfing. But alternatively, maybe her fruit power could have manifested as the handle of a wind-up toy where the target only remains a toy as long as someone keeps winding them up, or maybe she could give each toy a "battery" that needs to be recharged regularly to keep them transformed. Maybe in order to toy-ify someone she'd need to knock them unconscious or SPLAT them between a pair of cymbals like a wind-up toy monkey.
Law - Like many people, I think Law's abilities need nerfing generally. I say he should have never had any power that wasn't related to switching things around. He could have made the whole thing about Law needing to physically slice whatever he switches around without the aid of his magic room. Or at least, in direct response to the idea of "how do you counter his body switching?" I'd say, give it a time limit, or make it work until the people in question touch each other, or have it be dependent on catching your targets from behind unawares. Generally speaking though, I just wish his whole power could be re-written from the ground up.
Bonney - I think this is a fun one. Make it to where when Bonney strikes you she adds (or takes away) years of your age relative to how hard she hits you. You could also make the effects of her powers temporary as well.
Perona - Perona is fine as is. She caught the Strawhats by surprise for comic relief, but there's no reason she couldn't eventually be speed-blitzed if Usopp wasn't in the equation.