State of the champions.
It's terrible. AD has to many advantages over AP. Bruisers have are being given to much of everything. Crit chance needs to go. Mobility is still growing. Armor uniques only care about auto attacks not AD skills. There are huge item gaps (in terms of stat combos). The only counter to CC is 2 items 
, btw neither are AP/ANTI-AD. The only two anti-burst items are MR/HP and Armor/AP
. lifesteal is light-years ahead of spell vamp for no real reason. Also should mana still be a thing, or can we just scrap that mechanic already?
As a 5v5 game, the game is still getting better and is better than it has been in the past. Although, the list of viable team comps is still being cut short because of how poorly champions are being balanced against each other on an individual level. A team comp might have potential, but if you can't make it past the mid game without being massively behind or losing your nexus, then it doesn't matter.
i think this should cover this topic
? All of that health is made useless, and that's assuming they don't have kind of natural % damage or an execute.
, then you might actually see more champions in different positions. Also ADCs provide a consistent source of damage in fights, APC are burst heavy by nature, that doesn't mean they are more useful in fights though.
that can never be out of mana? The energy system has a similar purpose as the mana system but more limited.
being able to throw as many wiffs as he wants and no worry about not being able to try again as soon as CD is done. 
and unstoppable spam. 
being a helicopter more or less. 
and infinite ulti. And don't forget all the healers that can just then spam heals. Removing mana would require massive reworks all around to try to keep some balance in the game. Every game would just be like URF without the extreme CDR then and a lot less tactical. So yeah mana is more of a way to keep some balance just by preventing everyone from spamming everything everychance they get.
even need mana? Or why does 
have health costs, when all it does is force him to buy at least 1 spell vamp item?
pre-buff 
and i'd argue 
, because it holds them back to much or they just completely ignore it. Some of those use way more mana than they should or only notice they have mana when they miss that kill by the skin of their teeth. Look at all the champions who don't have a resource and are still "balanced".
, 
, and 
and maybe a forth if you don't count part of 
's passive . Energy, fury, rage, Hp, and energy are all resources. So yeah saying to only look at 3 or 4 champs is not much to look at. And Riven actually got nerfed hard in the past due to her constantly stomping early game.
The mana system helps hold back some champs that would just completely stomp the early game without something to hold them back. It would take a long time to rebalance the greater majority of champs that do rely on mana and all of the items that would need changed.