3 components champion designers should avoid
There are 3 components which should be avoided during champion design, since they're basically unbalanceable. They usually lead to a character either being over-or underpowered: 1.) Invincibility 2.) Self Revive Passive 3.) Hyperscaling
About 1.) Invincibility can be found in Fizz' and Vlad's kit, as well as in Elise's kit. Vlad already got nerfed long ago to make up for this sheer power, while kit kit was troublesome in several aspect "troll-pool" was forced the devs to break him down. Elise lost her invincibility and can now be hit e.g. bei Fizz's shark. Fizz hasn't been sverely nerfed, but considering the amount of complaints his Trickster is one of the next skills to be touched.
Conclusion: Invincibilty is troublesome, it either gets removed - or the champion gets severely nerfed. Please don't include this in champion (re-) designs
About 2.) How many years did it take to balance Anivia? 3? And still she's either underplayed or overpowered (those are not automatically opposites). Currently we're in a situation where she's underplayed. Fact is - self revive is incredibly powerful and most often leads to severe nerfs. Other champions who feature similiar passives are Aatrox who is not exactly up, but nowhere near competitive buffs are nowhere near the horizon: Mostly because the slightest buff might totally break him. While I can't really think of any QoL change that'ld break Zac he's in an even worse spot. He is one of the least played (and considered least powerful) champions and yet again we've a self revive passive.
Conclusion: Self revive is powerful - it's also frustrating for enemies to play against since you're not getting rewarded for an earned kill. Especially as long as Zilean and GA are still ingame these passive's will always restrain champions to not make a match 7v5 or 8v5.
About 3.) While hyperscaling isn't directly as problematic as the other 2 aspects it's incredibly hindering from a teamplay perspective. Hyperscaling characters usually are in a state near useless, or able to destroy a whole team. They get punished for roaming, helping teammates, or taking part in objectives by loosing a significant amount of stacks. Match usually start off as 4v5s and if they don't end soon enough they're won 1v5. Those champions are not seen in competitive play due to be being able to be taken out to easily. While they're sometimes seen in soloQ it's often a frustrating experience - for both teams. Most (random) players lack the communication to shut down a fully farmed Nasus - and as I already meantoned earlier: unless he is fully farmed he's not really a valuable asset.
Conclusion: While balancing wise it's maybe less troublesome, hyperscaling creates a toxic gameplay environment and solo-player culture. The experience usually is unfun for both teams and balancing these champions again bears the danger of extremes.
Final words: I believe that these are very present topics, considering the Sion & Cassio reworks, as well as Zac being the 2nd least played and Ziggs among the most banned champions ingame. We've seen these things in the past and I think we should learn from it instead of making the same mistakes again.