Here's a variant for your consideration:
Deadly Critical: When you score a critical hit against an opponent, roll another d20. If you roll a natural 20 on this second roll, the opponent's hit point total is instantly reduced to zero. For player characters, this means they must start rolling death saves. For NPCs and monsters, it usually means instant death; the GM may rule that certain monsters are immune [you will not be able to insta-kill Great Cthulhu with a lucky die roll]. If you are using the rule where the opponent can sacrifice a weapon or shield to negate the critical hit, the decision must be taken before the second d20 is rolled.
This rule is simple (in the spirit of 5E), applies equally to all (so even high-level characters will feel a little worried), yet won't happen very often, and won't instakill PCs (but rather bring them to zero hit points). Also, it's fun to roll dice, and crits become more interesting.