Being a teenager in the 90's, I remember when everything was EXTREME. So with that in mind, here's a few somewhat light hearted suggestions for making your OSR and retro D&D games EXTREME. Use with caution (or with characters that aren't meant for a long, serious campaign).
All damage dice "explode" if they roll the highest possible number on the die. When exploding, count the score, then roll the die again and add that. Keep going until no dice score the maximum. This applies to all sources of damage.
For example, if a character falls off a tower and takes 3D6 damage, the dice might come up 3,5 and 6. Roll the 6 again and score a 4. Total damage: 18 points.
All attacks cleave. If any attack kills it's target, the character immediately gets a free attack against another target. For missile fire, the new target must be in roughly the same direction of fire while melee targets must be within 10 feet.
If the new target is also killed, another attack is triggered.
Magic causes chain reactions. If a character is slain by a spell, another opponent within 10 feet will take 1D6 damage. This can trigger multiple times and targets killed by the chain reactions will trigger chain reactions themselves.
If no enemies are within 10 feet, the chain reactions will target friendly characters as well.
Any character scoring a natural 20 on an attack roll or saving throw, or a 1-5 on a thief skill, immediately recovers 1 hit point per level of experience (Confidence Boost).
This can bring the character over their normal maximum hit points, in which case excess points wear off at a rate of 1 per day.
Any character scoring a natural 1 on an attack roll, saving throw or a 96-100 on a thief skill immediately loses 1 hit point per level of experience (loss of confidence or embarrassing fumble).