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.
