This suggestion has been carried over from the Discord server, as the result of a conversation between myself, Laoni, Sten4321 and View619.
We were discussing how to keep light and darkness as an element of gameplay; so that while most of the time, yes, a character with Darkvision will be better in dark tunnels than a character without it, you will still need a light source on standby.
It would function like this; we have Darkvision and light function as in D&D 5E. Bright light, dim light, and darkness. No penalties for bright, dim light is disadvantage on perception, and darkness is disadvantage on attacks and perception. For characters with darkvision, dim light functions as bright light, and darkness functions as dim light. However, some creatures, such as our favourite lizard friends, would have bonuses to their attack and/or damage rolls while their target is in darkness - meaning the party will have to cast light spells and use torches to counteract this bonus.
So, instead of nerfing the characters we already have, it's giving the enemy an offensive buff. We did briefly discuss them having a defensive buff, such as additional AC, but that would punish the players who can't get close-range (if the enemy is high up on a wall, for example). In addition, when having to make the choice of holding your shield, which gives a +2 to AC, or holding a torch, which cancels out the enemy's +X additional bonus, could be a strategic choice.
This trait could be given to some monsters; for now, I'll just make an example. Shadow Strength. A creature with the Shadow Strength trait uses darkness to their advantage; when they hit a target that is in complete darkness, it deals an additional 3 points of damage. Note: when the creature is in darkness, not when the creature is unable to see.
To give as simple an example as my mind can make, we'll say that we have a human, an elf and a sorrak.
In Darkness: The human cannot see, but both the sorrak and the elf can see, because they have Darkvision. In addition, because the Sorrak is unseen by the human, if it attacks them, it will roll at advantage. Regardless of who it attacks, because all of them are in darkness, the Sorrak deals an additional 3 damage.
In Dim or Bright Light: Everyone can see and attack each other, and in Bright Light, the Sorrak has disadvantage on these attacks. Because they are not in darkness, the human and elf will not suffer the additional damage.
Elf in Darkness, Human and Sorrak in Dim: All three attack without disadvantage. Because the elf is in darkness, the Sorrak will deal an additional three damage to the elf; conversely, the human is not in darkness, and will not suffer the damage.