Earlier quoted context omitted.
To get worse than COVID you'd need a virus that takes a long time to develop symptoms and then has a high death rate. In that scenario there would be a lot more public fear and we'd have an easier time keeping people at home. So I'm still of the opinion it's harder to have a worse situation than COVID.
Another possibility: something that generates mild cold/flu symptoms but does long term damage that you won't notice right away. This would cause the "Bob had it and he's fine!" mentality to spread as it currently is. An example of this may be HIV.
There is also the minority opinion that some forms of dementia are due latent viral infections.
[1] You don't guillain barre syndrome takes years to recover and is absolutely traumatic.