[Item] Affix: Vulnerability

Vulnerability increases damage taken by 1%, can stack up to 20 times, and each stack lasts 30 seconds.

Could even introduce a few extra affixes in addition to the (current) first one below:

x% chance to inflict Vulnerability

Vulnerability lasts x% longer on targets you hit

Or if elites get a vulnerability affix as well, throw an affix on some torso armor,

You are immune to Vulnerability

Or throw in some punishing legendary weapon with the special affix,

Vulnerability stacks no longer have a stack limit

or

Vulnerability stacks no longer expire

or

Each stack of Vulnerability increases damage taken by an additional 3%

or something.