Like a sword/main-gauche (parrying dagger) combo item
Something along the lines of:
40 Attack Speed 30 Armor 30% Tenacity 20 Armor Pen
UPDATE. I forgot that people here take everything so god damn literally--they will argue specifically against the numbers that I randomly made up as an example instead of the concept.
Bhagswag2 votesPerhaps make it cost as much as IE. 20 arm pen and 30% tenacity are already amazing. Now you get 40% attack speed and 30 armor? jeez.
XxxLumberJackxxX1 votesIs like an item that builds from a recurve bow and two of the ruby crystal upgrades. An on hit dmg/heal based on your max hp
Bard R Friday1 votesTry Titanic Hydra.
oSEXYPLATYPUSo1 voteslol and we are bitching that 
are broken right now. and you wanna item like that.... wow just wow
but as far as off tank briuser duelist items....
just food for though....
TheDarkinLegend1 votesNone of those item give attack speed or armor, except for the gauntlet.
I want an item that gives defensive stats with attack speed. No AD necessary. The numbers I proposed were just off the top of my head. It was an example---it doesnt have to give armor pen, or tenancity, it could give attack speed, armor with some kind of passive stat
oSEXYPLATYPUSo1 votesNone of those item give attack speed or armor, except for the gauntlet.
I want an item that gives defensive stats with attack speed. No AD necessary. The numbers I proposed were just off the top of my head. It was an example---it doesnt have to give armor pen, or tenancity, it could give attack speed, armor with some kind of passive stat
Well you have alot of different items for those stats to have all in one item would be a bit over powered and be abused by all adish champs. As far as current items ghoustblade is the ideal duelist item which is being abused by marksmen
IcyPepper1 votesGee, I wonder who's going to abuse that item.
Boulderox1 votesno, the idiocracy is that there cannot be any GOOD ad/armor items because adc's will abuse them and become off-tanks as well. the above item is good, but that is an adc's wet dream, and an assassins pride and joy honestly.
showed us that, there shouldn't be.
That is the trouble with items, you can make them for one role, and another will find a way to break it.
EX. 
red smite top
mid
etc...
TheDarkinLegend1 votesThe item wouldnt give AD, only attack speed, armor and possibly tenacity and armor pen which im not married to, and those numbers were just random examples. I really hate how moron league players take the numbers as face value as the thing to focus on
Astôlfo1 votesSure, if you want it to cost like 5000 gold.
DarkRitual1 votesHey look, another ArPen item with stats many ADC's would love.