The point of the list of champions I provided is that they have ALL been very blatant issues in the past. They might not have built tank, but if they were in that state now, they likely would be exhibiting the exact same issues as the rest.
You want to take a guess at what the actual overwhelming pattern is? It's mobility. Out of the 14 prior champs 13 have some form of mobility, 92.8%. If you want to know what really breaks the most broken champs in LoL that's your answer.
Considering this is the only portion of your post that's relevant to the OP, I'll start with this.
Notice how in the OP I mention having mobility, utility, and %HP damage.
If a champion has mobility and utility, why do they need to have damage without building any?
If a champion has mobility and damage for free, why do they need significant utility?
If a champion has utility and damage for free, why do they need free mobility?
If a champion has all three, they should not excel at any. This is mostly the case. However, when you add tank builds into the equation, you now have champions that take a higher amount of damage than most, deal higher damage than most, and have the same if not better utility compared to most. And they're relatively low counter-play due to mobility. Point and click on the majority.
If you want to take a look specifically at Maokai...How long as it been since he's been a problem prior to his 'update'? Wanna take a guess as to what makes him the monster he is today?
Want an example that isn't %HP damage?
Irelia.
Jax.
Kass.
Nocturne.
They've all been huge issues at some point. Why? Because they get a significant amount of free damage that allows them to build primarily tanky and still be a threat. And defensive steroids, mobility, and utility on top of that.
Its not just %HP damage that's the problem. I never claimed it was. I said the issue isn't just IBG+Sunfire. Its the free damage loaded into so many different champions' kits now. %HP is just the most common, least creative way they persistently try to do it, and I'm sick of it being slathered onto most kits, as it only leads to problems with balance.
Lowering HP hurts all tanks in scenarios where they have to deal with true damage. It would make characters who rely on true damage far more powerful and lead to great imbalances in the game.
Except they now don't have to deal with %HP true damage. Tanks were fine in season 1/2 when HP was low, resists were high. Hell, that's the only reason they weren't a problem back then. Even Cho wasn't considered OP.
HP would still mitigate flat true damage. It would just mitigate it slightly less. You make it sound like I said "remove bonus HP from the game."
Besides, you can easily lower true damage that proves to be problematic as well.
You're also suggesting nerfing HP, a primary stat for tanks and tank's damage at the same time. Do you honestly think that's a good idea?
Does tank damage need to scale additionally with their HP? We all know what a trainwreck Tahm, Mundo (after E change), and Sej are when they're anything close to decent. Specifically because of their HP scaling damage. That simply doesn't need to be in the game.
Primary stat for tanks is resists. However, they trashed resists since seasons 1 and 2, so health took over. It used to feel just about as good building a cloth armor as it did building ruby crystal. They increased its value and decreased its effect substantially over the years. Which is Ironic, considering items used to have way more HP on them.
Health used to be the 'multiplier' for those who would buy wit's, FoN, FH, GA, hexdrinker, Atma's, etc. Many items had very high health to supplement those items that tanks would buy.
HP is a multiplier for resists, just like crit is a multiplier for AD/AS. At least it was. Now resists are only there to supplement HP, which is why you see such heavy HP builds. HP used to be ~1k-1.5k lower on average for 'tanks'.
That may have sounded tangential, but the point is that when resists are properly balanced so that an over-abundance of HP is not required, everybody wins.
Tanks are easier to kill because you aren't using an over-abundance of an uncapped multiplier.
Squishy damage dealers get more out of resists, making things like Wit's, GA, etc more worth buying, especially when it comes to sustain.
The above trade-off means ADCs have less damage, making fighters actually viable again via both build-paths and match-ups with tanks.
Supports get more defense out of their low budget.
Mids can actually build tanky AP if they choose without getting hotfix nerfed or feeling useless.
A specific class of champions should not be intrinsically impotent in a match because a specific champion is facing them. That's called bad balance. This is exactly why %HP skills should not exist. It should only exist through itemization, where everyone has access to it.
Winning at champion select would be a lot less common of an occurrence if the game were to be balanced primarily in this way again.