Nice. Thx for the feedback.
A) Your analysis is quite welcome. The goal is to make melee weilding dagger rogue more common. Yes there is a clear bump in power with dual weilding daggers. But at the same time, you must be behind your target. That often means that you are now in the thick of combat. The risk to be brought down fast is there. Also, it's a feat. The "archer" could still shoot from relative safety, use sharp shooter feat on low AC enemies, and gain a +10 dmg. At level 12, without critical, our dual weilding dagger rogue will do an average of +7 dmg against his target while the archer will do +10 within quite a safe distance... It would only be on a crit that the feat would flare into brigthness. Risking a lot to have a chance to do more. Is it that really superior?
So the backstab only option, as opposed to sneak attack is a nice throwback. And while that sort of positioning is possible in theater of the mind play like mine (and I have rules to cover it), it tends to only work if you're using a battle mat.
Having said that, as a feat it's less problematic (not sure why I didn't recognize it was a feat). And either way, it doesn't really impact the game that much. All you're really doing it increasing the sneak attack damage by 50%.
B) Again, The goal was to have more dagger weilding rogue. Maybe just weilding a dagger/knife would be ok. The enhance damage could be at the expense of the bonus action...
So this is a bit where these types of rules designs lose me. Mostly because I'm one of the "realism" types that likes things to make sense. To me it seems like what you're trying to design is the guy that brings a knife to a gunfight. No reason why you can't do it, just doesn't really make sense to me. Personally I think that it should be a heavy lift, because if there was a real advantage to it, then dagger wielding fighters would have been a thing. They were, but only because they didn't have a better weapon at the time. In addition, it seems to me that it would be cooler if they remained rare, but you have a character that put together a viable way to do it and was relatively unique.
So, leaving the reason why you would be a rogue focused on daggers to you, I'll see what I can come up with to make them better.
First, I like the idea that they do more potential damage, so an expanded critical range is fine with me. This is the biggest one, since sneak attack dice are also doubled on a critical hit.
Then I'd probably look at "common" dagger/knife maneuvers as inspiration (like your double backstab):
Perhaps a feat or ability that gives you advantage with attacks with a dagger while grappling.
"Twist the Knife" maneuver? Extra damage as a bonus action? Automatic hit the following round? What's the risk or downside? Not sure what I'd use for a mechanic on this one yet.
Threaten (I have this in my campaign) - If your attack roll would hit, you hold the completion of the attack and threaten them (blade to the throat type thing). You can complete the attack at any time, since you've already rolled the successful hit. The target can use their reaction to attempt to escape by making a Dexterity saving throw (the DC is your attack roll).
I have a parry option in my campaign (opposed attack roll), and finesse weapons have advantage on parry attempts. You can also use your reaction to parry.
A maneuver I have that ties into that is a riposte, where if you successfully parry the attack, you can use your reaction to make an opportunity attack. If your attack misses, they can use their reaction to make an opportunity attack. This is a great combo for rogues, though. Carry a dagger as your second weapon, Sneak attack with your primary weapon, parry an attack, and make the riposte and second sneak attack assuming other conditions are met with your primary weapon. Even better with a swashbuckler.
I think it was in UA somewhere, probably the upgraded flail weapon feat, with an ability called Batter. If you had disadvantage on an attack, but the second die would have hit, then they take a certain amount of damage. A dagger is quick, so I could see a similar maneuver allowing you to do a little damage where you would have done none.
That's a start anyway, I'll try to think of some more.
C) I do something similar. If you need a 20 to hit, and you do. you have to roll a crit confirmation. 11+ you get your crit. 10 or less, it will be a regular hit.
I went with the over 5 rule to include the confirmation automatically, and eliminate the extra roll. Most of the time, with your attack modifier, a 20 is a critical hit as expected. More importantly, it's not variable - the players know what a critical is and they've already rolled the die. They don't have to worry about a bad second roll. The main problem I have with the RAW is that if 20 is the only roll that will score a hit, then every hit that you make is a critical.
