Here's my personal opinion on it:
You can effectively compare Morgana and Kennen since they have somewhat similar kits, particularly their contrast between their long range harass and their up-close ults (Here I'll be using a mid Morgana for comparison). They are obviously different champions: Morg has her snare and her shield, Kennen has more mobility and stuns. Morgana has to get close and stay close in order to make the most of her ult, with only a long CD snare to keep them near. Kennen can get into the middle of the enemy much more quickly, and has a chance at multiple stuns besides. Assuming they will survive thanks to Zhonya's or whatever, Kennen's kit seems to be the superior at getting him into the enemy, disrupting them, and causing the most damage. Why is Kennen the less effective champion then?
Morgana's CC and damage on her ult is much more consistent. The ult just doesn't random decide who to ult: if you are in range, you will tether them. If you stay in range, they get the stun along with a large tick of damage. Kennen instead hits champions within his range randomly, so it isn't reliable at peeling, CCing, damaging, or anything that Morgana can do every time.
So what to change? We can look to another champion to help: Rumble. A while back, Rumble's Flamespitter was changed so that instead of randomly damaging people, it damaged in front of him in a regular cadence. Now, like Rumble, Kennen might have to be nerfed after, but we can do something similar for Slicing Maelstrom.
My proposed changelist:
Slicing Maelstrom
Duration = 3/4/5 sec --> 3 sec all ranks
new Mark of the Storm now applied every 0.5 sec
new Now hits all champions within range
Time between Bolts = 0.5/0.4/0.33 --> 0.25 at all ranks
Damage might need to be tweaked (most likely nerfed). I used Vel'Koz's ult as a basis, so the numbers may not be perfect, but this is the direction I think Kennen's ult should go. Regular cadence will make him more reliable, players can strategize with or against this, and it still has meaningful weaknesses (he has to get close).