I'm playing with the idea of adding a new house-rule to my sandbox-style game.
The rule is as follows:
If you succeed on a saving throw against an effect, that effect cannot make you drop below 1 hp.
I am hoping this will eliminate situations where an injured player rolls a 20 against a Dragon's breath weapon and then just goes down anyway because the half-damage eliminates them. In more extreme situations, it makes it so you don't instantly die with no chance if you anger a creature too powerful to defeat because the half damage might instantly kill you.
I also hope it encourages tactics both for monsters and players when dealing with a group of weaker enemies. According to the design principles, a large group of weaker creatures should remain a credible threat, but when a lightning bolt 100% takes out a whole line of them, regardless of their rolls or their cover (which is supposed to protect you from just that spell) that just isn't true anymore. With this rule, Goblins behind cover or who have advantage on the roll actually have a chance of surviving (with 1 hp, but at least they'll remain a threat). It will make these spells potent but not certain death.
However, I'd like to hear if anyone has tried this rule before, or sees any situations where this would be exploitable, broken, or "not fun".