The concept in its execution is a complete failure in my opinion. The idea is that even in death, his rage and power should be feared. The reality of his passive is it's just something to look at that makes respawning take longer. It chews through tanks, but most tanks have massive CC anyway. Combine that with surrounding teammates, getting any use out of your passive is often futile.
To me, I'd rather see a more complex take on the passive. Not in mechanics but in concept. The idea of being able to attack after death, just running around using basic attack takes no skill, there is no thoughtfulness. It should be a high moment for those playing Sion. A moment to redeem their death or extend their rampage. His passive becomes a joke if he is on the losing team, being 3 shotted by an ADC or Assassin then running around like a moron trying to make something happen. Sion's passive is effective if he has teammates around to slow enemies he is pursuing. Yet the current meta favoring early game champions with high AD burst, mobility and with most teams carrying 5 flashes - good luck.
The success of using your passive should not be automatic. Using Kog'Maw or Karthus, there is a skill required to get the most out of your passive. With Sion, there are too many variables that make his useless in most cases. Variables are not something Kog'Maw or Karthus deal with. They have a fixed amount of time with straight forward options. Those passives are high moments from defeat that when used correctly can drastically shift the momentum for their team.
Sion is a tank. He has high crowd control and his kit is built around initiations while maintaining his CC in a fight. The mechanics of his passive fail to extend and enhance the concept of Sion's intended design/fantasy. The concept itself is in line with that fantasy, but again the execution is flawed. The undead juggernaut, who even in death should be feared, isn't.
I think we have a case where numbers are not a fix. He needs to have gameplay/mechanical alterations to his passive. I'm sure we could all come up with ways to change it, but spit-balling isn't really constructive at this point. I doubt Riot would take any ideas to heart. There is too much feedback to wade through. To design your own solution would just be fuel for one's ego.
On a positive note, Sion's passive WAY better than Zyra's. That is one passive that just makes you feel worse about dying. It feels bad, does no damage, is too slow to use and fire. It just punctuates your failure.