Ok sooo. Here we go.
With this new pre-season we have already seen the shine of the assassins, which adds a lot of fun and troubles (vayne got buffed again ??). Anyway, Thornmail. I mean, seriously 80% of the AD champs are basically no more autoattacking.
And thornmail it's completely useless after minute 25. You can rush it, and sell it after for randuin's omen, deadman's plate. Anything is better, cause the damage returned is ridiculous compared to the lifesteal effects and armor penetration that are out there. And, there is deathdance now, which gives you a lifesteal effect that oblietrate thornmail returned damage just by spamming skills on the thornmail user.
The only item build addressed to make thornmail a little better is to build as much magic penetration as you can, but apart of abyssal scepter which can synergize with AP tank champions like Amumu, Rammus and Malphite, building a void staff or sorcerer boots on a tank is not really a deal. And, anyway, the role of thornmail to help tanks punish greedy adc's is broken. It's a fact you can't tank a fed graves, a fed lucian anymore, not talking about kha'zix and other new reworked assassins. Their sustain is just too good, they can fire freely on a thornamil with almost no drawback.
I think it's time to rework thornmail to address also skill based champs, exactly how Deathdance is helping them to shine: Lower the returned damage from auto attacks maybe and put in a returned damage for AD damaging abilities. Or make an updated version where thornmail procs Grievous Wounds effect.