sorry in advance for spelling mistakes and badly written sentances
in my opinion right now janna is not a lot more than a shield bot that has a team heal and anti-engage. with max cdr e with all the ap and shield scaling she has a 5.5 second cd shield, that gives around 50 ad, 20-30% attack speed, with 500 health or so, that lasts for 5 seconds. meaning its on a 0.5 second cd if it doesn't get taken down.
in my opinion, and in many other people's opinions its not enjoyable to play against, and it takes no skill because there is no risk just randomly putting your shield on an adc for no reason, the rest of the kit is fairly balanced, but why would you work hard when you can basically just e people and fairly ok.
I think there should be some type of risk for playing her, and for shielding at the wrong time. I think its supposed to be used for the damage part of it less than the shield part of it.
she has also been in power for very long and i think it would be fun for a little bit of a shift in the meta.
I think the cd should be higher at 45%, not by a lot even 1 or 2 seconds are enough.
the shield strength should be lowered by maybe 20-30 base value, which lowers it with all the other shield and heal powers respectively, maybe also a bit of an ap scaling nerf, but you do the testing it might not be strong enough in case the scaling gets nerfed.
and lastly (and I am being generous here) buff the ad the shield gives, so it will be the focus of the ability instead of the shield itself.
if i have to put it into numbers ill guess it will be something like this.
shield strength: 70 / 105 / 140 / 175 / 210 --> 70 / 100 / 130 / 160 / 185
shield cooldown: 18 / 16 / 14 / 12 / 10 --> 18 / 17 / 16 / 15 / 14
shield scaling: (+ 70% AP) --> (+ 65% AP)
bonus ad: 10 / 17.5 / 25 / 32.5 / 40 -->10 / 19 / 28 / 37 / 46