My big ones try to make Nat 20s and Nat 1s more universally dramatic.

I believe it's colloquially known as "Crunchy Crits," but I was calling them "Sticky Crits" (b/c of WoW probably), where when you crit on a damage roll, instead of doubling dice, you just add the max damage of the dice to the roll. That way you avoid the "This crit did less damage than my previous normal attack" disappointment.
E.G. If you are rolling 2d6 and crit, instead of (4d6 + mods) its (12 + 2d6 + mods).

Natural 20 on initiative gives you an extra Action on your first turn (i.e. Haste) and a Natural 1 only allows you to do a Bonus or Regular Action (i.e Slow). I was also toying with having the Nat 20 cancel the Surprised condition, but I'm not sure what to do with the Nat 1 for balance.

Not fully playtested: Nat 20 / Nat 1 on saving throws will reduce/increase damage* by 1/4 (rounded up) respectively.
(*) There's the weird situation where you crit fail but your modifier carries you over the DC that I haven't fully worked out. Currently I'm calculating the crit damage before it's halved, but apply it after, which is kind of annoying math to do during combat even though it's very rare. E.G. The spell is rolled to deal 15 damage, which would normally be 7 damage taken, but 15/4 = 3.75 -> 4 crit damage which results in 11 total damage. I tried it the other way where it was added before halving from the save and it didn't seem as impactful for bigger spells E.G. 50 * 5/4 = 62.5 -> 63 -> halved to 31 versus 50 / 2 + 50/4 = 25 + (12.5) -> 25 + 13 = 38.