Well, if it's soooo awsome, why does nobody buy it? If you do, that's how we spot you are noob and kill you. or at least that/s what i do.

thornmail is situational, i get it sometimes, and i see some get it as well. sometimes the pre items are the deal, and thornamail recipe is to clench the build. a slight buff would be nice, but its good that way that it is, not great(sadly).

ive got a 4th option? keep everything but reduce the % to 15 but.. it increase based on a calculation of the champ level AND/OR summoner level.. just a thought, or increases in % from the attacking champs level, just a thought, but ... yah..

It already scales based upon the level of incoming damage you are receiving, why make it scale twice?

Not a bash, or flame, just curiosity to see what else that idea has behind it.

Fixed since bloodrazors dont stack anymore.

There are plenty of items that are very situational and rarely used. Thornmail is one of them, but you may be better off suggesting a NEW item designed for a caster's damage mitigation, since you seem to want to not use the armor stat. Thing is, this is a tank item. It has high armor to help you understand that. If you want to make a sub-2000g armor item into something that scales for AP instead and gives the same mitigation a tank has to invest over 4000 gold to get, I want to change Deathfire Grasp to scale with armor instead of AP and grant +25% spell damage mitigation...

Point is, armor stat is fine and balanced. Mages can stack it as well. Thornmail, as is, is fine. Sure its less used, doesnt mean it needs to be completely redesigned.