How should Senna get nerfed?
Pick Rate: 27-33% Win Rate: 52-53% Ban Rate: 49-52%
Okay, I think it's pretty clear Senna is going to get some kind of nerf. So what should get nerfed?
For the passive, I'd like to see her need 25 stacks to get the bonus range and crit chance and I'd lower the crit increase to 12.5%. This would lower her safety and sustain a little at the end of the game and let her fall off a bit. Basically without kills, this nerf would require another 2.5 minutes on average to hit another threshold.
For her Q she already has yuuuge mana costs and a 15 second CD, but basic attacks lower the CD. If her players discover Manamune/ER then this might be an issue, but for now they seem to be going lethality. My only recommendation here would be to make the beam a little narrower. It'd make her poke and healing a little harder and dodging it a little easier.
With her W, the root is 1.45 - 2.25, so I'd like to see it last from .5 - 2 seconds and up the CD on the skill by 1 second. This way you have a little less poke in lane as well as shorter CC in lane since I think she maxes this second. In teamfights it won't matter so much. Considering she has so much utility, what she gets should be a little weaker overall. Jack of all trades and master of none sort of thing.
With her E, I think I'd change the duration to 3 - 8 seconds. I don't hate this skill, but I think there should be more pressure to max this sooner.
As I see it, these proposed changes would hit her early game most of all and it would require the player to make some interesting choices. There's more encouragement to level up the W and E a bit sooner to get back to the old levels of utility but late game you are still pretty close to where you were. With everything together, the early game is a bit weaker without hitting her late game too much.
Ashe worked as a supporty ADC because she just had one main thing, CC (and nebulous vision 95% of the player base won't make use of), and a big flaw, being a lack of mobiliy. Senna has infinite stacking, 2 forms of sustain, CC, team mobility, and a big offensive/defensive ult. Considering she has so much going on, I think the most fair thing to do to her is make her a later game champ by thwacking her early numbers but not her later ones.
So TL/DR, I think her early should get nerfed with her Q needing a little more skill to use. She's got so much going on I think it might be best to just slow her down instead of taking a nerf bat to her numbers. With the bot lane XP nerfs, this is in line with what riot is doing for the lane and lessens the likelihood of the other team having to deal with a fed support/adc.