As I said, numbers can be changed.
Considering it's a worse passive than 
in most regards and worse than 
's passive, I'm not sure you'd make Thornmail really stand out with a stacking %Damage Reduction boost.
As-is it's also worth noting that 
essentially reduces all auto-attack DPS by 15% right away to everyone nearby, where if Thornmail were to work on a stacking debuff system it would also target auto-attacks/sustained DPS more than anything else, causing both Thornmail and Frozen Heart to occupy essentially the same exact niche: making 1 the best and the other a strictly worse version.
We could make less stacks, or make it so that ranged attacks produce 2 stacks.
Ranged assaults producing double stacks is interesting, given defensive items like 
have pretty significant anti-melee properties and Thornmail as-is punishes melee champions a lot harder, as it's more difficult to switch focus when the area your AA range lets you smack things in is a tiny fraction of the area ranged attacks can smack something in. That idea is probably worth perusing.
We could also make the power scale with bonus armor, to maintain the current sinergy.
%Damage reductions innatly scale with armor as-is.
Causing %Damage reductions to scale more with armor is like scaling attack speed from an item based on AD. %Damage reductions and armor work very well together.
I think that the spike thematics still remain, you can imagine spikes that damage your equipment, so it deals less damage.
How are you damaging your opponent if you're dealing no damage?
In LoL, Fiora can also obliterate someone with a ruler, Lee Sin can OHKO someone while wearing boxing gloves, and Mundo can kill someone with a briefcase.
Spikes damaging things without damaging things makes no sense.
Also, there is another thing we could do: make it a true tank item, changing the effect so that the attacker deals reduced damage to everyone except the one with thornmail.
Interesting idea.
While aura's are usually a lot of passive power that's difficult to feel, such a dymanic to an aura effect could break that mold and offer a more interactive and interesting item.
That said: if the item is offering a ton of defensive stats regardless, you're not creating much of a dynamic with focus by the aura not effecting the tank. In practice it's still the same general deal as or 
. The guy wearing it is tanky, his buddies have a little more durability as well.
So it doesn't make a tank immortal, but makes targeting him first a bad thing to do, as it should be.
Generally speaking targeting a tank first is a bad idea regardless of the situation.
Also: generally speaking you want your opponents to make that bad decision and feel like they must focus you down, not the other way around. As a tank in LoL, you're job is usually managing the aggro in the real-world sense and actually convince your opponents to focus you: usually from being really disruptive, other times from being so amazing at zonning opponents they really have no choice.
The tanky targets that don't want to get focused are mostly Fighters, as they use defense in a way much closer to how assassins use mobility or marksman use range: as a means to survive long enough to kill things, and not getting hit helps them survive longer.
In this sense: "Don't focus me" is something more helpful to champions like:
that are largely forced to build defense to survive with how their kits operate, but still want to crush their opponents.
Thornmail also currently fullfills the "get off me bro" dynamic, which is likely why has a decent track-record of purchasing Thornmail in competitive play.
