She was my favorite ADC as well. What they did to her ult scaling is just completely stupid. Getting AP makes it do more phys damage, getting AD makes it do more magic damage. Like... why? Why would you make it do damage according to the inverse of what you stack? So you can't go black cleaver / last whisper for arpen, because then it will do more magic damage from the AD. Go void staff, it deals more phys damage. It just makes absolutely no sense.
She used to have less steroids than other ADCs, in exchange she had good lane bullying and a great teamfight ultimate. Now, she's just bad. Lane bullying is all she has. Her range is mediocre, her scaling is awful, and her ult does trashcan damage late game. Even if you stack AD items, the damage on W is still terrible. 130-140 magic damage after i hit somone SEVEN times, when I crit for 800+? Tons of adcs will have somone dead before those stacks even get to 7.
Just make her ult scale with AD again, and take off the stacks. Make her W something more fun than "the more I hit them, the more I do a miniscule amount of bonus magic damage".