The next meta shift
If you have not been keeping up to date with any pre-release notes or playing any PBE then you're in luck. I'll catch you up with major changes short, fast & simple. The developers are changing up some dragon features & how they interact with the map, altering some support items, adding 3 other support items for different roles & adding 2 more lethality items. While I reluctantly agree with the dragon & support changes, adding 2 lethality items inadvertently creates a downhill resistance funnel against everyone other than tanks. So before we begin- As the title implies, this is going discuss the the ups & downs of the assassin class, why I believe the Meta starting off in the new season is purely because of those 2 new lethality items & why I believe having the AD assassin class become meta once more is not necessarily be a good thing.
First lets talk about the present strength of an assassin's build before we dig into some detail about these 2 brand new items.
Their original build of 3 lethality items at the same price of 2900 gold: 
21 lethality, 55 damage, 10% CDR + passive,
18 lethality, 55dmg, 10% cdr, 40 O.O.C.M.S. + active, 
18 lethality, 55dmg, 250 HP, and a spell shield. Is already relatively revolving around Zed, Rengar or kha'Zix since they are typically the stereotypical AD assassin. For 8700 gold, you receive 57 lethality, 165+ damage (because of Duskblade passive), 20% CDR, 250HP, 60 roam move speed, and a shield active.
After these few main items you will see some take or ignore Edge of night & go with Black Cleaver, Death Dance, LDR & or a Mortal Reminder. In absolutely every sense, an assassin class is given the option to expend their entire kit to get to their target. The problem about that though, is that the assassin's kit has 3 opportunity windows to do instant action as to where every other class requires time to pull off any action which casts a window for ability interruption. Now I don't have a problem with most AD assassins until I realized that most AD assassins have more than one ability to apply these item's damage with multiple times under a second and that they're able to wipe out someone completely counter building. Although the assassin is meant to fall off due to scaling, a good assassin will always have the opportunity to find the hole to oppress other lanes so your team has leniency to winning. This is why roaming was passively buffed by making tower's plating a little more robust in the next patch & why the rune "water walking" was finely tuned.
The main issue I have with AD assassins is how the idea of these items are solely catered around the removal of resistances when tanks were once the identity of an assassin's counter. Since the alterations of Armor items are already live, anyone who has to build armor and health becomes less and less favorable and even much less viable to play. After awhile you end up seeing the shift of building more damage but sustaining much less because they have nothing else to fall back on. If you don't understand, let me grant an example to explain. Say an AD assassin has both a Black cleaver (+40 AD) for 3,000 gold & a LDR (+45 AD) for 2,800. Combined, they can reduce someone's total armor by 59% for only 5,800 gold. Because armor is relatively cheaper, works best with your kit and their entire team is ad, you chose a resistant build. You buy a frozen heart that grants 100 armor for 2,700 gold and a Sunfire Cape that gives 60 armor & 425 HP for 2,750 gold (5450 gold combined).
Of these two, how long do you think the resistive build will last? Well, lets just say that you have only 160 armor and the assassin only deals the items as damage upon ability cast. For only 350 more gold, the assassin will ignore 110.4 armor (if gets all cleaver stacks before death) on 1 ability, output 84AD, and deal only 34.4 damage from your health bar. Now although removing 34.4 HP from your 425 HP does not sound like much, it means a ton when the enemy can do this kind of damage immediately in any window of opportunity; We also have to remember that we're playing around with numbers solely based on the items that the stats grant the champion. If we were to implement this type of math with champion stats, the outcome would favor the assassin because of their versatility against "heavy beasts" that move slow and act sluggish.
Moving on, lets talk about the updates, new items and why they too cause a problem and would conventionally be worse than the previous damage. First up, the alterations. Duskblade lost out on its vision removal passive, but undergoes a damage increase by 5 while staying at the same price of 2,900 without changing legality damage. Edge of night looses out on 5 damage and 8 lethality, but gaining 75 HP having only 50 damage & 10 lethality and 325 HP in total while activation is now a passive. Next the new items: The first item is named Umbral Glaive which grants 50 damage, 12 lethality, 10% CDR and has the ability to instantly remove placed traps and do triple damage to wards for only 2,400 gold. The second item is named Sanguine Blade. it grants 50 damage, 10 lethality, 15% lifesteal, an increase of attack speed and lethality if found within a 1v1. for 3,000 gold.
Lets just say for a second that an assassin decides to buy all of these items. In total the assassin would have 315 damage + 71(+8 if solo) Lethality without Rune/mastery pages. 30% CDR. 325 HP. 15% life steal. Have the potential to increase attack speed by 40%-100% based on level, have a freebie spell shield, and have great overall roam power. With this build, they now oppress the living daylights out of anyone. What do I mean by this? Well, the majority of champions get near 100 armor at level 18, but this game is a race and the majority of times someone will be oppressed or picked on so they can't get experience points. At most, the assassin will start to fall off once everyone hit about level 16, but that is a huge gap of 10 levels filled with windows of oppression. In fact, there is a 4 level gap which the assassin can achieve if played correctly.