What if Thornmail's Thorns were active-centric, not passive-centric?

The problem with it being passive is that it CANNOT be strong, or it's OP/Toxic/etc. It gives your enemy no "windows of opportunity" to play around it; your enemy can only itemize against it with stuff like Mercurial Scimitar, Death's Dance, or Maw of Malmortius, which results in an arms race that Thornmail always loses at (no one ever gets %MPen with Thornmail).

Back in DotA All-Stars, the item Blademail was of similar design, but its thorns effect was based on an Active component. You activated the item, and it gave you 100% thorns toward ALL incoming damage (including spells) for a short time. The obvious counterplay was to bait the active and make your opponent waste it, then attack while it was on cooldown.

I have to wonder whether or not our Thornmail might do better if its Thorns effect was tied to a limited-duration active (it'd still only reflect damage from auto-attacks, not spells). That'd allow it to deal much more damage, but gives your opponents opportunities to outplay it. And if your opponents are smart enough not to attack you while it's up, you can still use it to deter enemies from attacking you while you make an escape or try to get into a good position.