Nat 20 Initiative

if you roll a nat 20 for initiative, you get advantage on your first attack roll.

I figure that's about like critting on an initiative roll, so when PCs/NPCs crit on initiative, they go first, and I give them a full additional action during their first turn. Extra attack, or spell (following casting rules, of course), dash, disengage, or what have you. Otherwise the 5e limitation on crits (only to-hit) feels kind of lame. It's a real buzzkill when someone rolls a 20 and I have to tell them it's not an automatic success and it doesn't mean anything special.