I don't think it's even possible to have a meta where every champion is viable
Edit: I don't think it's even possible to have a meta where every champion is equally viable in League of Legends.
The title is kinda stupid because a meta, I believe is the most effective tactic available? or something that works better/best in the game. And if everything works equally well, there wouldn't be a meta theoretically. I could be wrong.
Anyways, here is what came into mind that would make said meta impossible: I am self aware that this is a narrow, tunnel vision example but take a look at Ashe and Rengar. Let's exclude grasp of the undying Tank Rengar Top for this example to work better Keep in mind that when I mention "one shotting", it's referring to one shotting squishies, champions that generally don't stack defensive items.
Ashe is literally incapable of one shotting because of the nature of her kit. Even a pretty fed ashe that's an item above everyone else isn't able to 100-0 you in 1-3 seconds. Rengar in the other hand, specializes in one shotting and is capable of doing so even if he's an item behind because of the nature of his kit and the lethality items he utilizes. Lethality takes advantage of you not building armor and so even if you are ahead, you can get screwed if you only build offensively.
Now let's say Riot nerfs all sources of damage by whatever percent and nerf one shotting sources (skills, items etc) so one shotting ceases to exist.
Since Ashe is a dps champion that gets stronger as the fight gets longer, she'll become way more viable than Rengar who's kit tells him to one shot when he can't anymore.
I mean people are already doing this in a very smaller scale by taking resolve as a secondary. Usually it's on champs that has any form of DPS, and not on one shotting assassins. They are essentially self-inflicting a nerf to their overall damage while increasing their likelihood and duration of survival which grants them more DPS. This is evident when you find two sparring champs early in lane and one of them takes resolve as as secondary and the other doesn't.
** If you haven't caught on yet, I think there are champions and items-design wise that are keeping other champions from being equally viable. And if you nerf the oppressing champions (and items), you are indirectly buffing the champions that were oppressed and making them meta. So the oppressors become the oppressed and the oppressed become the oppressors. **
If this still doesn't make sense, I'm saying if you make one shotting champions unable to one shot, you are indirectly buffing DPS champions that thrives in extended fights where they don't get bursted down quickly. It would be a buff to Yasuo, Vayne, Ashe, Tristana, Twitch, Kalista, Swain, Nasus, Ornn etc.
So if burst damage isn't a thing, you would then find people trying to find ways in keeping these DPS champs stay alive even longer- which makes shielding supports meta again. Literally, if you can't burst down (assassinate) any DPS ADC fast enough, Soraka, Janna, Sona's viability is going to skyrocket. Yeah even Sona because you would be keeping everyone alive longer while being safe from being one shotted yourself.
Which is why nerfing all sources of damage greatly favors some champions over others.
I just remember throwing up after watching the WE vs C9 games in worlds because of how cancerous a Kogmaw was while being peeled by an entire team. I actually believe that C9 was a better team mechanically but WE just had a better read in the ardencenser meta which was brain-dead.
I feel like one side is always going to be crying because of the other side when it comes to assassins (burst damage) vs DPS squishies/Shield supports/ Mages. As assassins are only viable when they are able to one shot because they heavily lack DPS, while the other side becomes useless when there is practically no counter play to an assassin's burst. Fyi a janna support usually doesn't buy Zhonyas- it's not some universal item for every squishy that's not an assassin. What do you do? Change the design for these assassins so they do more dps instead of burst? Then people are going to cry about the design change and how the assassinating skills/kit was iconic for that champion. Zed isn't zed anymore if he doesn't burst but deals DPS like a yasuo. How do you fix this.