Make Senna an AP caster
It makes more sense for her to be an AP caster. Her abilities would be better off using AP damage than ad damage, considering that she has long auto attack animations and long cooldowns on each ability with weak ratios due to the utility of her kit. AP items have a lot of CDR so it would help alot with the CD, mana issues, and be more intuitive to play with.
Also, It makes more sense to balance her healing on how many mists she collects than giving it an AP ratio. That way support senna does more healing (since it collects more mists per minute) and laner senna does more damage.
She would really have an easier time using AP than AD. The best AD items on her rn, for damage, are lethality items, which are burst damage items.
Thats just my opinion though.
P.S. rito for the love of everything holy please make her e less crippling to use.