I’ve always liked exploding critical rules. Additionally, in 3.5 D&D I often played with a triple critical rule where three natural 20s meant you automatically killed the target. Admittedly when I was a player I loved this rule but as a DM it often threw wrenches into my gears (one of my players was nicknamed “Dice Jesus” for a reason)!

Thinking about those rules recently made me realize I wanted to write a houserule to bring the fun of exploding criticals to 4e. I couldn’t simply copy the same mechanic though, I had to make something better suited for 4e.  I came up with two possible rules I might try (although not at the same time!):

Exploding Critical – On a critical hit, roll again. If that roll is a critical hit add one bonus die of damage to your total number of bonus dice. If your weapon or implement doesn’t have bonus crit dice, add 1d6. Repeat until you do not role a critical hit.

Who doesn’t love to roll more dice? Who doesn’t love the thrill of crazy good luck? Really this rule will most often only add a dice or two to the bonus critical dice they’re already rolling for their magic weapon/implement without heavily swaying the game. It just gives that satisfying extra “umph”.
Note that it requires a critical hit, so if they have crit enhancements – like critcal hits on a 19 – then this still triggers. If you have a player that crits low,  then next houserule might be a better choice for your table.

Maximized Critical – On a natural 20, roll again. If that roll is also a natural 20, in addition to your regularly maximized damge, do not roll your bonus dice. Instead they are also calculated as if you had rolled max damage.

This is a more controlled, if slightly hefty approach. It requires a natural 20, not merely a critical hit. It maximizes the bonus dice, but doesn’t add more dice. Easy for the DM to anticipate because the DM doesn’t actually need to raise their damage expectations. A player could roll max damage on their bonus dice by pure chance – so getting to maximize occasionally it should not shake your numbers too badly. I would use this for games that tend to have deadlier enemies for combats with more punch.