So, I tested Senna out a few times now (great work), and i stumbled upon three things that i think could be improved (nothing about balancing, just quality changes):
1. I think it would be useful if you could ping your passive stacks on Senna (like Veigar can), to better communicate with your teammates about powerspikes.
2. It would be nice if her passive cooldown (the part where if she hits an enemy two times she deals bonus damage and gets a soul) on enemies was actually visible (like eg. Sejuani). This would make it easier to know when trading is actually worth it and who was already hit with it in a teamfight.
3. I believe her bad attackspeed scaling should be more directly adressed in her passive: "taking longer to fire" in my opinion doesn't neccessarily mean bad AS, but just a long AA-Animation.