Lots of systems try this.I'm sure I'm not the first to have this idea but a quick search of the forum didn't lead to any matches, so here goes...
Thinking about called shots lead me to the idea that a critical hit should allow the attacker to say what their hit does.
For example a critical hit on a cyclops might wreck its eye giving it disadvantage on attacks (because it can't see).
Or for a multi-limbed creature it might sever one of its limbs.
Of course monsters with crits against PCs would have a similar opportunity, though I might make it more broken limbs/blinding etc giving some disadvantage on movement or attacks.
Double damage by itself (especially when a combat is just starting out) can seem a bit underwhelming, perhaps this is too much?![]()
One idea might be allowing you to swap out extra dmg for a condition check.
Pick a few conditions that maje sense and let crits choose one of them in place of damage.
All would need vet process for balance. Does this favor twf vs two handers or bows by giving more rolls? Does it disfavor spellers more?
However, i am not a fan of (very opposed to) further empowering the die ( Locking more and more cool stuff into dumb luck and away from character and concept ) so it goes against one of my core principles for design.