I think RIOT should do an examination of damage in the game without the intent to implement some new-fangled cool idea.
Basically, they need to start by revisiting true damage. Let's say that one champion in the game has too much true damage... well that means that in order for the game to stay balanced, everyone has to do more damage in general. Visit the worst case scenarios fist back off a little on damage that bypasses defensive measures.
For example, let's say you have a champion that does percent health damage (percent current, percent missing, percent max). Think about what that means. That means that they are bypassing the defensive measure of buying items that give more health. That's not a good thing. So tone it down, back it off.
Once RIOT does an exam of the worst offenders, they can get down to balancing the damage for all champions. Because once you back off on the true damage and percent damage and penetration damage options, etc., etc., then you can balance the other damage in the game to be reasonable.
For example, people have a lot of complaints about 
. They talk about his mobility, or his windwall, or his ult putting him outside turret range, or his double crit chance, but what I don't see people complain about very often is his 50% armor penetration on ult. But if you think about it, Yasuo has to have 50% armor penetration on his ult in order for his damage to be balanced with all the rest of the outrageous amounts of damage that champions do.
For example, ADCs have a line of items that moves them to ridiculous late game damage. This is by design. But the disturbing part of that is that it is by design so as to conform to META. ADCs have to have this kind of damage to conform to META and that makes damage every out of control too. So this has to be re-imagined in a way that doesn't conform to META. The multipliers on damage can be scaled back and raw AD can be brought back as the main source of damage increase. Multipliers include: attack speed and crit chance (which incidentally happen to exist on the same items because why not?), and armor penetration and percent increase based on health of enemies (which incidentally happen to be on the same item). If you think about it, an item like
can't be balanced unless it does have stacking damage multipliers because the other ADC options have stacking multipliers. So ADC itemization is a disaster (and, of course, it enforces conformity to META). Maybe ADCs want this conformity because they want to be able to play any of the ADC options in more or less the same basic ways. Well, that's bad for game. RIOT should force them out of their comfort zone.
Once all the more unreasonable forms of damage have been scaled back, and the rest of the champions get scaled to deal their appropriate amounts of damage... I think the balance team should be able to take care of the rest. Defensive and offensive options should come into better focus.
One more thing, changing defensive options is definitely not the place to start in fixing the damage problem. The reason is that defensive options are too easily bypassed in the first place by things like true damage and percent health. So if defensive option get buffed it aggravates the situation of having defended against most champions but not really gained any defense and the major offenders. And the only way to balance that difference will be to increase damage across the board. And then, we will be right back where we started and complain again.
For example, let's say RIOT buffed armor. Every armor item now has more armor. well, armor doesn't work against true damage. So armor didn't do anything to stop the major offenders.. it just nerfed everyone you was trying to keep up with the major offenders in the first place. So that necessitates a buff to everyone who does physical damage. And you have a champion like Yasuo who can ult for 50% armor penetration. And he's going to be one of these champions that will need a physical damage buff. Do you see? There's a vicious cycle that happens here where you can't balance the damage in the game because maybe there is one or two (or a few champions) in the game that do percent health true damage.
Let's look at 
. When Fiora was reworked they wanted her to 'be a threat'. And the cool idea to make that happen was percent health true damage. And maybe you think, "okay, but it's just one champion that can do that..." And that's where the designer falls into a trap from which he can never escape. Because once you have even a single champion that can just ignore any and all defensive measures that would be built against it, all champions in the game have to be balanced around it. And as long as that exists, the damage can never ever ever be balanced. it just isn't possible. So more champions come out or get reworked and they have to have some sort of true or percent health damage in order to 'be a threat'. If they don't, then they can't be balanced in the game because if even one champion gets to ignore all defenses, then all champions will eventually need to have their damage buffed. And it wasn't that long ago that RIOT decided to buff 
with his percent bonus health true damage. And if you think about it, something like that eventually inevitably had to happen in order to keep Cho'Gath's damage balanced with the rest of the game.
So because buffing armor won't stop something like percent health true damage, using a buff to armor as a starting point won't fix the problem in the long run. In fact, it will eventually just make things worse. And everyone will be left scratching their heads and wondering what can be done to fix this intractable problem with the game. And the problem is really just that there was this one really cool idea that designers wanted to have on one champion in the game.
Is there a place in the game for percent health true damage? IMO, there is. You could put it on a summoner spell like ignite and balance it that way. But you cannot give it to one champion in the game as your cool idea to make that champion 'be a threat'. Now, I'm not saying the if RIOT fixes Fiora, then all their damage problems go away. Some smart designer might read this and think that that's what I'm saying, but the problem is too big now. The damage has creeped in at the seams and there is a long list of champions doing true damage, and champions doing percent health damage, and champions with ridiculous armor penetration, and what not and all these sorts of things are delicate. If defensive itemization becomes pointless against any one of these champions, the system breaks down all over again and we get damage creep all over again. So RIOT has to do an examination of every champion and every item in the game and fix all of the biggest offenders first if they want to get the damage creep back under control.
What about champions that are supposed to instantly delete enemies? Someone might ask what we should do about a champion like 
. I shake my head at you. Have you understood nothing I've said? There is no problem with Le Blanc. There is no problem with instantly deleting enemies. That's a red herring. You are getting distracted by some mistletoe dangled in your face. Le Blanc doesn't do percent health damage or true damage or anything of the sort! She could buy 
or 
or 
or 
but she doesn't have any of the problem damage issues in her kit. The objection isn't that Le Blanc instantly deletes enemies, it's that all champions in the game are doing too much damage too quickly. So RIOT can't fix the problem by going to champions like Le Blanc who are accused of instantly deleting enemies and 'balance' them. That's not the starting point. RIOT will inevitably be forced to revert that sort of change in order to re-balance them with the game.
Thank you for your time if you were brave enough to read my wall of text.
TL;DR
The key to fixing damage creep in the game is to do a comprehensive examination of champions that get true damage, percent health damage, armor penetration, magic penetration, and the like in their kits. The game is forced to be balanced around champions that that ignore defensive itemization. If even one champion ignores defensive itemization, all champions are forced to become balanced around that simple fact.
and 
to break 
and
in the first place makes it blatantly obvious how out of hand damage has gotten.
A champion thats supposed to be based around his ult for getting kills.
when 
can get it done at a much safer distance
