Now that Kennen is in the dumpster with pre-season changes, can he finally get the rework he needs?
tl;dr this champion was designed for League in 2010 and hasn't kept up with mobility creep or the high damage meta. His win rate in NA is currently 45%.
His kit doesn't synergize well and doesn't allow him to do his job as a team fighting champion. His mobility is too weak and doesn't allow him to get on top of high priority targets and he doesn't have the survivability necessary for staying in range of high priority targets if he does manage to get in. He's forced to build two items to cover these two weaknesses, but as a result, he does no damage. This means he needs to reach 3 items to actually be effective.
Klepto covered this weakness to some degree by expediting these items. Without Klepto, it takes him too long to obtain the items that he needs just to barely function. The old Nimbus Cloak also helped to cover his mobility issues, by allowing him to get on top of his target faster when he ults, but now that's gone.
Scaling champions needing items to be strong is one thing, but a champion should not need 3 items just to perform their basic role.
Update his kit to something that works in 2020. If nothing else, increase the cooldown on his E and give it something like a mini-flash (i.e. Pikachu Up+B in Smash).