i agree she needs a change but i think what they are doing isnt the right way
for example the passive AA damage based on bonus MS?
that is a poor way to change her
i think what they should have done was this
make her passive not give bonus AA damage instead let her AA mark them with her passive for 2 seconds, making Janna's allies run up to them at +5% move speed (still keeps her passive)
E changed to 18 seconds at all ranks but AAing an enemy champion will reduce the CD by 10/20/30/40/50% with an internal CD on per E shield cast (making her have to shield then actually AA for the better CD, but keeping CDR from going overboard because its a 50% reduction in its current max CD so with CDR the -50% is less effective at how much it reduces on say a 16 second CD [-8 second], vs the full 18 second CD [-9 seconds], this will still result in a buff late game to the CD IF she is AAing an enemy champion, exposing herself instead of sitting in the back)
W i would change to allow janna to extends the slow by .5 seconds per AA she lands on the slowed target (again encouraging her to attack and not just run) up to 3.5 seconds, but the slow duration is reduced to 2.5 seconds (slightly shorter duration on slow in return for more offensive play making the slow last slightly longer) [she wants to AA twice for max slow duration]
these changes would encourage janna to AA now and then in a fight vs being a shield and CC bot without making her an AA every chance you can for the bonus damage from passive that the current changes will cause, which will likely result in more viability for janna as an adc herself vs as a support due to self peel, self shield+bonus AD,+bonus AA damage based on move speed items (
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would likely result in a bonus 50+ damage per AA, not to mention her bonus ad from shield and a 
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resulting in a very strong late game build that runs fast has self peel and shields would result in her being a better ADC vs assassins than any other ADC due to 2 self peels (Q,R) a shield (E) and a slow/move speed (W)
the current direction they are going with janna looks to be changing her to be better at adc than support for effectiveness and add on a support like lulu for extreme tanky adc due to the shields, life steal, self peeling, support peeling, and lulu ult
i just feel the current changes would be making her more of an adc than a support, make her have to AA some for her CDs and utility to be more effective NOT for more damage, otherwise she shifts for becoming an adc
