Hello again, all!
I am unsure if there will come a time where daggers will be an iconic rogue weapon in the future. As such, I am creating a homebrew rogue archetype to sort of tide me over until then. This is merely a beta. Forewarning, I did not do any number crunching or anything of the sort. I went with what felt okay as features for the archetype. Obviously, to even use the features to their fullest, you must be using a dagger. As to where it's too OP or underpowered, let me know and we will see how we can improve it.
I will try to explain the reasoning for each feature.
Daggermaster
Improved Critical: Starting at 3rd level, attacks you make with daggers can score a critical hit on 18-20.
Reason: Fighters get a crit range of 19-20 and eventually 18-20 with ALL weapons. An 18-20 range for daggers only seemed reasonable, even with sneak attack hovering about, licking its lips. This has also always been a typical thing daggers get.
Dagger Tosser: At 3rd level, the range in which you can throw your daggers increase to 30/60.
Reason: Sometimes you just wanna throw a dagger, especially if it's magical and returns to you. The increase in range should help some of you, especially if you like to fight from a distance, but feel that a crossbow or a bow doesn't exactly fit your character's theme, like mine. Daggers normal range is 20/60.
Dagger Flurry: At 9th level, you can attack twice instead of once whenever you take the Attack action on your turn as long as both of these attacks are made using a dagger.
Reason: Here is the rogue's "Extra Attack" feature, but limited only to daggers and pretty late in the game. Granted, with that extra crit/throwing range, you're encouraged to throw multiple daggers. Three if you TWF! I am not sure if the feature is either OP or UP. You be the judge.
Hidden Blade: At 13th level, you can distract your foe with a series of flourishes, creating confusion as to where your attack will land. Using the bonus action granted by your Cunning Action, you can misdirect one enemy within 5 feet of you. Doing so gives you advantage on attack rolls against the target until the end of the turn.
Reason: I really liked the Arcane Trickster's mage hand ability, but wanted to go with a less-magical approach by using the rogue's innate guile and wit (Cunning Action) to gain the upper hand on his foes, should he find himself toe-to-toe with them.
Counterstab: At 17th level, whenever you are missed by a melee attack, you can spend your reaction to make an attack using a dagger against the target that missed you.
Reason: An ability at this level has to be something pretty damn cool. An out-of-turn attack is exactly what a rogue would like. Namely, the Assassin's Creed series comes to mind where you would counter an enemy and gut them. In this case, the target misses you with a melee attack and you have to spend you reaction to make an attack of your own. Think of this ability as the offensive version of Uncanny Dodge. Useful, but you still want to be wary in the event you have more than one enemy trying to kill you.
I feel that 5e rogues were designed to be more utility than DPR, with sneak attack supplementing a suite of situational and non-combat class features. Giving a rogue subclass nothing but DPR combat features looks highly overpowered without even number crunching.
The most striking (hur hur) imbalance is Improved Critical. Since you get to roll all the damage dice on a critical hit, including sneak attack dice, this is just too much, too soon. I'd either go 19-20 for the entire subclass career, or switch levels with Hidden Blade, linking the 18-20 range to using cunning action. More on this below...
Dagger Tosser and Dagger Flurry seem reasonable given their limitations to daggers. I'm not keen on the idea of giving rogues Extra Attack on general principle, but then lots of rogues MC fighter to gain that perk and more, so why the hell not.
Hidden Blade defeats the purpose of imposing limitations on sneak attack in the first place. The rogue is effectively gaining advantage on every attack against every enemy. Seems a bit much. Compare to Oath of Enmity, which at least has a 1 use/short rest limitation. Despite being a much higher level class feature than Oath of Enmity, I'd impose a uses/frequency/target restriction. Taken with the above suggestion about linking Improved Critical to cunning action instead, and you need not choose between the two subclass features.
In fact, I'd rather see Hidden Blade as the level 3 feature, usable once and lasts a minute against a target you declare, and recharges on short rest. Fluff it up as saying enemies are wise to your maneuvers after killing one of their buddies with it. Then make Improved Critical your level 13 ability, granting an 18-20 crit range with cunning action.
Counterstab seems okay.
Overall, I think this subclass would balance out by replacing any of the features with something not expressly made for stabbing. Or if you're enamored with all these combat features, at least grant expertise to performance when the rogue is juggling knives. 
Hidden Blade+Improved Critical+Dagger Flurry=52.200625% chance of getting at least one critical.
That's over 50% chance of doubling ALL you damage dice for one attack, and thus your insane amounts of SA dice.
Yeah but the chances of missing with advantage are low enough that that's a negliable concern.
Well lets see. Starting with two attacks with advantage, the odds of hitting but not critting on the first attack and missing on the second:
Assuming base 50% hit chance and the 18-20 crit range, you hit but not crit on 11-17. So the odds of rolling in the 11-17 range on at least one and not rolling 18-20 on either would be:
1-.25(the odds of missing)-.2775(the odds of critting)=.4725
The odds of missing on the second attack is a simple .25.
So the odds of hitting but not critting on the first attack and missing on the second attack then are:.25*.4725=.1181, or roughly 12% chance of wasting sneak attack if not applies to the first hit.
When you have 3 attacks it becomes trickier.
You could have a non-crit hit and 2 misses, or 2 non-crit hits and a miss.
Non-crit and 2 misses=.0295, roughly 3% chance
2 Non-crits and a miss=.0558, roughly 6% chance.
Total chance of wasting SA if only applied to crits or last attack if non-crit hit=.0853, roughly 9% chance.
So once you have 3 attacks with advantage, it's a small but still noticeable risk of wasting sneak attack if not applied if you hit with your first attack. If you get 2 non-crit hits first, then your odds of your last attack being a miss are nearly doubled from the previous attack. So this proves that waiting to apply sneak attack only on a crit or your last attack does have a significant risk of not getting to use it at all. Obviously this is less of an issue against lower AC targets, and more of an issue against higher AC targets (although against very high ACs it becomes less of an issue when your chance of critting is greating than your chance of hitting but not critting).
It still givess you slightly more than a 1 in 4 chance of critting with each attack, for slightly more than 1 in 2 chance of getting double SA damage as long as you're willing to risk a 10% chance of doing no SA damage. That's most definitely OP.
Thanks everyone for the input! And yes, I did try to think of fluffy stuff to put into the archetype, like performance with juggling knives, but I couldn't really think of anything too useful other than it's actual use (stabbing people). I do not aim to make it an overpowered archetype and I know the rogue is more utility than combat-oriented. If anyone has any ideas on how to use a dagger in a non-combat situation, I am all ears! It could definitely use some interesting fluff!
Given the math, I would have to agree that taking the crit range down to 19-20 would probably be balanced out and possibly even swap Hidden Blade with Improve Critical.
As far as Hidden Blade is concerned, we could rewrite to where you use your bonus action granted by your Cunning Action to trigger the ability against a target of your choice (within sight or 5 feet) and if you attack a different target, this bonus is lost and it recharges on a short rest. Also, remind me because my group confused me: A round is a span of 6 seconds, right? Or is it actually a whole minute? I'm trying to think of a proper duration for Hidden Blade. Hell, we can make it last a number of rounds equal to your Dexterity modifier to make things simple! 
Advantage against a single target for a number of rounds equal to your Dexterity modifier. If you attack a different target, this bonus is lost. Recharge on short rest.
I have to get ready for work for now (ugh) but I have a half-day today so I will definitely check back on here and will try to think of other features. I shall return!
Idea on editing Dagger Flurry and removing Counterstab
Dagger Riposte: Whenever you are missed by a melee attack, you can spend your reaction to make a melee attack with a dagger against the target. You do not deal Sneak Attack damage with this attack, unless the target had critically failed to hit you.
CrimsonAssassin wrote:
Slightly off-topic, but this thread made me think of something: the description of the Attack Action says that if you may make multiple attacks you can do so with one action. Does that mean you roll and resolve each attack indepenently, or can you do so all at once? I.e., serial or parallel?
This matters because if you do it in parallel you can indeed go "crit fishing" but waiting to see if either attack is a crit, and then say, "I'm using my Sneak Attack on that one."
Is there anything in RAW that either makes this valid or illegal?
Serial.
"...you can attack twice, instead of once, whenever you take the attack action." Interpret this as, the second attack is an option, but only after making the first attack.
Nevvur wrote:
Unless your swinging two weapons at once, you cant hit at the same time. As far as I can read into your querry though, thats just all about what order dice are rolled in, or what grouping you should roll them in. I would still do one at the time, cause if you kill a creature with 1 out of 3 attacks, and you rolled/declared all three attacks at once on it, could say you just continue to hack away at a dead body.
For sneak attack though, you can declare it after rolling the two hit. One of the advantages of duel weilding as rouge, if you do miss with first attack, you can try for the second (bonus action) attack to get it in.