Decrease AS and buff MR.
There are a number of problems with the meta, and they all stem from the same place: too much burst per second. No one gets a chance to breathe; it's like playing with nothing but Assassins and Divers with the rare overloaded AP Bruiser (tank). There is no point to playing if even a single person on either team allows any member of either to snowball and the snowballing player is playing remotely well. The answer is to take away everyone's ability to nuke eachother, which lies in the sheer frequency of strong autoattacks and the complete lack of agency of AP burst.
It's important to note that AS bases aren't the problem - champions should be able to comfortably last-hit in lane, nor should abilities that are supposed to give AS get nerfed - but free AS gains per level for every champion currently bog down the meta past laning phase in a way that shouldn't be possible. The likes of Xayah are allowed to accomplish so much in such a small time window thanks to how their kit works; same as many Assassins as the extra autoattacks break the champion's kit by letting them miss nearly every skillshot and still get kills. %AS per level - as well as the free AS at level 1 that so many champions have been given for whatever reason, mostly to Tanks and Assassins - deserves to be removed entirely. Compensate by slightly raising base AS on champions that lose the most, and everyone with an AA reset deserves extra attention so that they're not overpowered.
MR is the other issue; or rather the lack thereof as magic resistance is piddly in comparison to how much damage mages can do. Hexdrinker + Treads should be a last-ditch effort to not get nuked, not any given AD champion's default. The fact that so many champions have such little MR otherwise that they desperately need both of these items depending on enemy team comp or even just a single snowballing carry is utterly sickening. Also, has anyone seen Conqueror? I get that it's preseason and all but it is completely busted on mages! They don't deserve to have SV be any given tank's best item against them right now - if anything they deserve Force of Nature to come back!
People can and should compensate for the lack of AS per level with Lethal Tempo as that is the much healthier rune for carries than the likes of Conqueror or even PTA. Keystones get a lot of flack, but if not for the sheer amount of damage in the game already then they would be much more healthy and balanced as they all genuinely have loads of agency. But Riot, it is irresponsible to let your flagship game rot like this. The asian market only plays your MOBA because it's funNY and accessible and you get to be an anonymous goon for an hour at a cafe, not because the game's genuinely a great time. That's why people on these boards downvote literally everything: they know just how enjoyable the game is supposed to be and anyone talking about small-scale changes for current gameplay is trampling on this tombstone we all come visit and pay respects to every so often.
Tombstone, Riot. Because you've killed gameplay with burst per second. Bring it back or lose your attention.