Regarding Janna winrate
So I've seen a lot of Janna threads recently pop up yet none of them are reasonably constructed in terms of what they actually want besides "56% win rate hehexd nerf now".
Anyways..
Let's start with pretty much the one thing that has put Janna on blast in recent days, which is Ancient Coin. Before I get into Ancient Coin I want to also point out that Janna win rate was around 51% before coin was buffed so this right now this is a clear problem with itemization. She wasn't given any direct buffs or anything prior to this to plastering her with the idea of nerfing her instead of the item is like fixing a problem with solution B rather than A.
Couple of reference in regards to win rate:
http://www.leagueofgraphs.com/champions/stats/janna
https://na.op.gg/champion/janna/statistics/support (Korea, cause NA doesn't work for some reason)
http://champion.gg/champion/Janna/Support
Anyways Coin was buffed in order to give slightly more gold in comparison to Spellthiefs and Relic Shield, so in comparison to the influx of gold most enchanters scale well of items especially with ones that impact the game highly such as Censer, Redemption, Mikaels etc. so Janna in this case is building these items much more sooner than other supports because of her gold income.
This was also the same issue that Ivern had seen when he was first released, he had a very easy and safe clear and could counter jungle easy then he would rush both Redemption and Locket which were relevantly cheap and highly impactful. Eventually his clear was nerfed and remdemption and locket were given distinct identities and latter nerfs, so rushing those doesn't feel as good or impactful.
It's even come to the point where the efficiency of coin is even being used on champions like Sona, Nami, in higher solo queue, though Sona case is a little different because the mana refund on assists is good for a hungry mana champion like her because she can easily pick up a lot of assists with her auras.
Just to look at a few.
This creates a disparity between the two choices as one offers way more value in terms of getting to their power spikes sooner.
Traditionally supports were always gated by gold income which made their items cheap enough to compensate, but it was also a role focused on being impactful without gold champions like Alistar, Blitz, Leo, Thresh wouldn't really build coin because they're still useful and contribute without needing their items to do so.
TL;DR Nerfing coin would bring Janna win rate down so she can't spike as soon as she is currently doing, without actually nerfing Janna.
in particular come to mind.
double nerfs, never forget (or repeat).
/ 
auto proc on nearby minion deaths, because post-laning phase, when you're grouped with your team, damaging the enemy still procs spellthief, being around minion deaths still procs coin, but relic becomes really hard to work with due to mages/ADCs clearing so goddamn fast.
completly off the edge
passive on the coin, It's like having 2 Jannas in 1 make your life 10x easier when they run towards you and they get free gold, for just being there, it also allows her to be aggressive cause she can just use her bird and then flash behind the enemy and the adc can still have the passive by running toward her,enough to delete them. 
is a pretty trash gold income item it's so pitiful to not get 650g at 22 mins when the other gold items already reached it by then.
support take ancient coin. She didn't do much laning phase, but outside of laning phase she was able to one shot our adc because of how much money coin gave her.
, but what it leads into.
+; the issue is that now passive supports have good gold income along with access to cheap items.
up in price, since it is almost exclusively coin users buying them.
