Senna's kit is pretty nice and fun to play, but it's the exact opposite to play against. Just to rant, I'll go over a few issues I have with her kit and point out a few fixes.
I don't like the way she scales with her passive. I'm fine with her passive being the way she gains AD and range, but what I don't like is that it gives you crit chance. This opens up too many issues where she can end up with full crit AND lethality items at the same time. She ignores armor AND deals double damage on every auto. That's not really fair.
The attack speed on her auto's is nice, that helps her kite and be able to stay in fights. I don't like how it scales up though. Eventually she can land a single auto on you and get to kite for free endlessly. I'd lower that movement speed by a bit and remove the crit chance on her passive, or at least restrict her crit damage in some way.
Her Q isn't unique in that it can heal allies AND damage enemies, but it's unique in that it does both at the same time(unless another ability does so that I'm not aware of) It's range is pretty crazy, even if the cast range is her basic attack range. I'd remove its ability to target towers and wards, and cut the range down about 15% to make it feel less oppressive in lane. Currently its difficult to effectively dodge it, because even if you're not in range for her to target you, she'll just point that gun at literally anything that can be targeted and hit you instead.
Her W is a mini Varus ult with the delay shifted around. Period. Theres literally no other way to look at it. The range of it is already too damn high, and the animation doesn't always look like it ends where it should. It feels like it goes an absurd distance for such a strong root. I'd either lower the root duration, or its range. The AoE is fine as is.
Her E is Pyke's E, but she gives it to whoever else wants it too. :| Talk about recycling. I want a completely reworked ability.
Instead, I'd want it to be a single aoe cast around her.
Upon casting, herself and all allies hit are briefly stealthed (less than half a second) and they become wraiths for the duration of the ability. Wraiths all look the same, and have their health bar obscured for the duration of the ability. Up to 3 seconds max.
Wraiths can still auto and cast spells, but have their identities obscured. Meaning you'll still see the windup for a darius q around him, but it will be the wraith model with no healthbar. Attacking or taking damage wouldn't break the wraith-form either.
(probably not balanced as is but i'd prefer this unique ability to a reused pyke spell :/ )
Her ultimate i'm fine with. Maybe lower the width of the beam, but i'm otherwise 100% okay with it. It's rewarding to land the narrow center of the beam for damage, but you're still making an impact with the shielding regardless.
Yeah. Gimme your thoughts or rant amongst yourselves in the comments or whatever. I'll be over here seething at this senna dealing most of my health in a single auto and chasing me down endlessly :D