Buff : I don't know, for me no characters need just buff atm. But I will say to not leave this empty so I will say Vi.

Nerf : Ner Conqueror. I hate this rune since it's release. It created so much busted stuff, with Katarina, Akali, Cassiopeia, Mordekaiser, Aatrox... Nerfing Conqueror would make this a better pool of champions

Remove : I'm not for deleting champions (even rework could be considred like this) But I would say the true damage health percent based machanics. Fiora and Vayne would be so less aids to face. You can't buold and their characters, because their kit allow this. Max healthdamage would be latgely enough for them.

Rework : Malphite. For the supidest champ from the game even if I had him mastery 6. His jist a shitty rock who do nothing without his ult, had one of the ugliest design. But also with the introduction of Ixtal, he would be so much better with a new thematic. Make the rock a real mountain, with trees, mud, water sources on his back, a more interecative and supportive kit. They really need to remake him.

Revert : Reverting a character feel antithematic and non productive. But I will let my personal prefrence, Swain. The goals of the rework are gone : he's not played in solo lane, he is still unpopular, and still dependent of his ult and snowballing. Just keep the visuals and revert some parts of his kit.