I spammed senna for the first 2 weeks of release and racked up about 75k mastery on her. Here are my thoughts on her:
-She is way too safe for a champion that scales infinitely. You should be able to shut her down early to halt her scaling in its tracks, but with the combination of her AoE CC, camoflouging shroud, and ridiculous range, you can't do that.
-Other champions that scale very hard (Vayne, Kog, Twitch) have extremely short range and punishable early games to offer big windows to shut them down. Why doesn't Senna have that too?
-Her Q being able to target literally everything except thin air will need to go. Senna can use Piercing Darkness on fruits in the jungle, on wards, towers, etc, for a free heal that can save her in too many situations. It's also pretty cancer when she snipes you by placing a ward and Q'ing through it when you thought you were out of range. Maybe change it so Senna can only q through ally champions, minions, and maybe towers.
-Her W is pretty much a mini Varus ult. The root is pretty long with a generous hitbox, and for a peel/cc ability it does a damn good bit of damage. The one or two seconds you have before you're cc'ed usually isn't enough unless you're playing Pyke or Yuumi. Perhaps lower the damage, change it so it detonates instantly but only hits one person, like an ahri charm almost. It feels terrible to have your ally get hit by Last Embrace, and then you're dead too because you couldn't get away from them in time. (It's almost like playing against zilean.)
-I really do have a problem with her E. It's a strange ability, but one that will be completely busted in highly coordinated play. What's not to love? You don't know who's coming at you, the 15/0 Kha'Zix or the 2/6 Draven. While this ability does SOUND cool, it's extremely frustrating to play against. Why does it make you unselectable? It's like an AoE Vlad pool that speeds you up instead of slow you down, that also hides your identity.
If Rito don't do something this patch, just wait until Senna is inevitably picked in pro play. She'll see her nerfs soon enough then.