Since Riot is looking at Rylai's

Why not scale the slow based on it's owner's %current HP?

For example if you are above 66% health, it is weaker. If you are between 34% and 66% it is the same. If you are at 33% or lower it is a little stronger than live. That would give it more kiting and outplay power when you are low, but when you are healthy you can't bully and CC as well.