This sounds quite reasonable; possibly line-of-sight could be taken into consideration?
Though I'm trying to think of a good solution for stealth specifically, as in avoiding detection entirely; sneak attack in 5e is much weaker than backstab in 2AD&D, so it's less likely to oneshot an enemy. That's why I like the "silencing dagger" suggestion, though I have no idea how well it would work in-game. I'd love to be able to play a party optimized for stealth/assassination and it to be a viable playstyle (hold the cheese). :P
Line of sight and noise (or lack thereof) should be taken into account. Could have npcs talk to each other, if one does not answer then the others start looking to see why. There should be frequent patrols in some areas too.
For avoiding detection then maybe sleep poison?
Would probably be difficult to get/make a lot of it so would have to save for strategic targets. Sleep poison could allow the next attack on the target to auto crit. Or maybe could have some special attack for after knocking out a target.
The closest thing 5e has to a real "sneak attack" (besides rogue's Sneak Attack) is attacking while the enemy is paralyzed or unconscious:
Any attack that hits the (paralyzed or unconscious) creature is a critical hit if the attacker is within 5 feet.
Larian could do something with this? If the target is unaware (you are hidden and the enemy is not hostile) then any hit is an auto crit. Combined with rogue's sneak attack, this could so serious damage (1d6 dagger + 2d6 sneak attack)*2 for crit.
However, they'd definitely have to improve enemy's ability to detect sneaking characters or else sneaking would be waaay too overpowered. Add a circle around the enemy for hearing, combined with their sight cone.
Sneaking is already too overpowered. If they improve it then your ideas could definitely work.
