The correct Janna nerf.

Nerfing Janna has become something of a meme for a while now, particularly with reddit users who are convinced she needs to have her ultimate and shield nerfed into the ground. Often you see people pointing to her consistently high win rate, despite a history of nerfs.

She's also compared to Lulu - they have fairly similar kits, lots of CC/peel and a strong shield, yet Lulu sits at just above a 50% win rate right now.

Janna's problem isn't her shield or ultimate - it's the passive. Consistently, Riot refuses to nerf her passive in any way. Janna's passive offers her entire team constant, free peel without requiring the player to think at all. Just exist as Janna somewhere in the team fight. You can be out of mana, out of spells, it doesn't matter - if you exist, you're providing constant peel for your team. Any player who picks Janna can exist, providing her entire team a very influential amount of MS.

Now that her AA range has increased, and you often see Janna running in with W to initiate, her passive's free strength is even more influential, allowing your team to follow up and get on top of the target that much faster.

Riot should heavily consider removing her movement speed aura, and leaving her passive as free magic damage on auto attacks scaling with her MS. Removing that much free, hidden power will expose her true power level. From there, we can continue buffing or nerfing as needed, though I'm confident she'll require buffs after removing that passive. Riot has a history of not knowing how to handle Janna, and randomly shaving off numbers until she's considered under control again.

For reference, back when Janna had a global MS buff passive, when it was changed her win rate dropped a few percentages (below 50%) and she was subsequently buffed.

TL;DR - Remove Janna's ally movement speed aura passive. Buff the interesting parts of her kit afterwards if she needs it.