AD Assassin item concept
I thought of a new item but for that I needed a new component item with Attack Damage & +Movespeed to make it work.
Cursed Spear 1100g
[ Long Sword + 750g ] +15 Attack Damage UNIQUE Passive: 10% Cooldown Reduction UNIQUE Passive: +5% Movement Speed
Cloak of Stars 2800g
[ Serrated Dirk + Cursed Spear + Stopwatch + 200g ] +40 Attack Damage +10% Cooldown Reduction UNIQUE Passive: +10 Lethality UNIQUE Passive: +7% Movement Speed UNIQUE Active: Grants invisibility for 1 second (120 second Cooldown).
This item is aimed at roaming/ganking and outmaneuvering/outplaying rather than raw damage, hence the lower AD & lethality. More skill expressive and less burst/stat-checky. Not an ideal rush item unless you can weave around skillshots and wards. Naturally, the active synergizes with 
. Some might be against giving assassins more invisibility & more ways to proc duskblade, but this has lower offensive stats for the price and no defense capabilities except for the MS and active. Also, a normal stopwatch gives you 2.5s of stasis where this only gives 1s (but you are able to move while being unseen & untargetable).
"The unseen blade is the deadliest" - 
. You could argue Morellonomicon's redesign was an attempt at an assassin AP item but mages are still using it heavily just as much as AP assassins, which means so far there's literally no AP assassin item.
or 
. Perhaps they should make a pure AP+pen item (and maybe 10% CDR). A lot of mage items either have HP or mana, having (another) pure offensive item might make it worth the risk. Maybe something like