Thornmail rework is perfect for every tank, except Rammus. I have a solution.
So I see a lot of shit about the new thornmail, but the majority of complaints are from Rammus mains. The counter arguments in general seem valid. Tanks shouldn't do free damage for tanking, but they shouldn't be health pots. Grievous wounds on thornmail fixed a lot of these problems.
Now only Rammus used the damage on thornmail offensively. Because it synergized with defensive ball curl. Rammus before the thornmail changes would deal the damage return in addition to his own w damage. So I propose to do one of two things.
Either add scaling damage return to Rammus w active. Such as 5/10/15/20/25%, but remove the extra 5% armor ratio on w active, or increase the % armor damage passive from 10% to say 25% to make up for the change in DPS. Either change would probably bring Rammus back up.
Aside from him, no tank is intended to use thornmail for damage. It was made to counter A SINGLE healing item. Now that healing is very stackable, grievous wounds performs the job better.
/Tldr: buff Rammus not thornmail, if you use it for damage, you're using it for something that wasn't its intent.