I'm a little sucker because I want a Janna rework almost solely to freshen her up. She could definitely be so much more with an update to match the new identity of her lore. Her model is old and has wonky physics by today's standards. Her ability icons haven't ever seen a change.
I think she's a great support, and I really enjoy playing her, but I wish she was designed more like Rek'sai or Bard. The abilities of those champions have simple, yet useful effects that still allow skill expression and in some cases a fair bit of creativity. They also are clear in their effects. Janna's E gives invisible stats and her passive is in a similar place. Although visible, and now allowing Janna to interact with it a bit more with her positioning relative to her team, the effect isn't too noticeable.
I'd like her to remain a defensive oriented support, especially since they made her name mean guardian. Since Taric is the Protector's hunky henchman that design space might be kinda occupied though. I think a lot of people would prefer it if she weren't so damn good at her job. She can peel with incredible reliability. This is her major strength and her niche, but unfortunately makes her pretty uninterracrive and in some cases utterly frustrating to play against. It can feel like Janna is able to counter your engage without fail and with minimal effort.
Her theme also presents a few issues when it comes to the amount of creativity it allows. How out there can you get with the effects of her abilities while still being able to convince the player that wind's doing all that? There'll also be areas of obligation. "Wind magic" has become synonymous with movement speed boosts and displacement CC. Without those kinds of effects people might not really feel like they're playing a master of moving air around. So the pressure to make a cohesive and interesting kit while still staying true to the obligatory wind mage theme becomes present.
She might be a difficult champion to rework while still checking all the boxes. 