If you want to make a new AD Assassin item...
...Then the thought behind it should be "how do we make assassins more effective against squishy champs?" instead of "how do we make assassins deal more damage?"
The current idea Riot has for the Assassin Dagger is actually surprisingly archaic and uninventive from a design perspective. As has been mentioned in a couple other threads, this item basically serves as a true AD version of DFG (as opposed to the long compared BotRK). This item is basically a big "Here, have a pile of yummy stats and damage!" with almost no real reason why anything is the way it is. Riot could have gone so many different direction with this to make it a premier assassin item. They could have made it target base stats (i.e. Nightfall deals % base health damage, or "abilities ignore 20% base armor") which is naturally strongest against a squishy, or they could have given assassins incentives to be even more assassin-y in their build and play style (i.e. "basic abilities deal 10% bonus AD as additional physical damage" or "gain 10% increased AD" or "striking an enemy with an auto attack or damaging ability increases your AD by 20% for 1 second, stacks up to 3 times, 30 second cooldown"). Instead Riot gives us a relatively lazy, easily abused, stat booster type of item.
Riot, please don't ship Assassin Dagger, ever. Talk to your players about what AD assassins actually need and want and how to achieve that in a balanced way instead of blindly forging onward with this item.