Hm i want to argue about a few points.
Lifesteal on the existing items are rarely any cause for problems, even with 200 AD you won't heal more than 20-40 health per hit with 10-20% lifesteal, it's only when boosted with stuff like 800 crits that it becomes a problem, then the marksman needs like 13 hits to heal up full which is very doable on minions or jungle creeps, it's probably too much.
Especially when you factor in ruunan's hurricane.
Grevious wounds however is a tought topic.....but i honestly think riot should just remove it all together in all but a very few rare cases and balance remaining healing around that.
Trying to balance healing around being countered by grievous wounds will just make said healing insanely strong when it isn't outright countered.
Crit is in fact not RNG, or it's a controlled RNG, if you have 40% crit chance and "miss" twice then you´ll have a hidden boost of 80% crit chance for the third auto, easily tested and true.....but crit users need to be tuned down a bit because they hurt other squishes too much and the best anti tank builds are rageblade builds anyway.
Glass cannons are fine but they need to have doable counterplay that doesn't have twice the cooldown of said glass cannon fire rate.
Enchanters are basically supports with shields/heals and CC, which happens to be good against divers and assassins which naturally makes them ideal protectors for the marksmen who supposedly lack tools to do that.
Do note, marksmen with said tools are usually the most problematic ones or the ones who becomes problematic the most often.
Difference between G-tier and F-tier.....honestly some champs just get gutted to get them outa the meta waiting for a rework at times, we can't really help that but the others who "fall out" are usually not so bad unless hard countered.
I´ve seen Skarner played successfully on Diamond streams, despite a long time reputation of being useless.
Ryze.......you could write a thesis on Ryze and his many flaws but i doubt it gets anywhere.
Many divers are in fact fine but their counters and targets are overtuned instead and many of them lack proper tools do well in teamfights when not in a ideal situation from what i can tell, due to LCS plays having happened they have been balanced around said ideal situations happening when actually played which is part of the problems.
Cassio and Azir, or maybe Aurelion sit in the same boat if looking for mage equivalents
Assassins....is a hot potato, but my own belief is that some of them indeed got healthier in terms of playstyle they either lost on the "fun" factor or they didn't really become any better .....looking at Akali and Fizz here among others.
So to summarize.....some marksmen need to be toned down or reworked due to having things they should not, supports could stand to have more actual weak points or windows of oppertunity after laning rather than a nonstop 40% health barrier or a point and click stun (Lulu i see you) and some divers need help to endure non optimal teamfights and maybe laning as well.
CDR, % health damage and crit damage could all stand to be toned down.
Jungle could use shorter respawn rates and lower EXP to force less gank spams.
idk lower the ap and give %AS based on AP
target abilities hitting turrets with reasonable damage ofc (work with lux passive but no Q/E/R)