Lemon Law: If less than 2 sessions in, most things are up for change about your character. After that, if you've used the mechanic more than once, it's considered permanent.

The Rule of Cool: Want to do something outside of the rules? As the GM, if it's cool, negotiate about making it work for you.

Secret Death Checks: All death saving throws are made in secret by the GM.

Resting: Long rest must be in a safe place, or hit points are not returned. Safe is defined as no passive enemies within a mile. GM will make the determination.

The Disengage: Disengaging from combat is too hard. Disengage is still an action, and prevents all attacks of opportunity. However, it can be used as a bonus action as well, and grants disadvantage to all attacks of opportunity. Giving more ability for people to retreat!

Improved Crits: The first damage die set is maxed, and you only roll the second set. Ie, if the damage is 2d6, you get a crit, the first 2d6 is max (12), and then you roll 2d6 and add that normally.

Level 1 Feat: You get one.

Ability Score Increase or Feat?: Should you take an ability score increase or a feat? Why not both? ASI for a stat at 4, 8, 12, 16. Feat anywhere ASI's are normally mentioned.

Healing Potions: If you drink a healing potion in a bonus action, standard roll applies. If you drink it in an action, max is returned, and if you use it over the course of a short rest it is maximized and the first hit die used to heal is also maximized.

Monster Conditions: Bloodied. Half HP is bloodied.

Minions: From D&D 4th Ed. Monsters that are added that are way under the characters power level, and their HP has been updated to 1 hp, which means that if they're hit, they die.

Expanded Stride: Free Draw or Stow as part of a Stride action. Drawn from PF1.

Item Identification: Common rarity loot that is equal or lesser level is automatically identified.

Monster Identification: Type, and subtype on success. Every 5 over the players can ask one finite question and get an answer.

Hero Points - Stayin' Alive: When using a hero point to stay alive, gain 1 HP.

Hero Points - Alter the Narrative: On a scale, 1+ hero points can be used to alter the narrative.

Surprise Round: They exist. Two actions instead of three.

Attacks of Opportunity: Only triggered when leaving reach, not when moving around within reach.

Grabbed Enemies Can Be Shoved in Any Direction

DC for Aid is the Lower of 20 or the Main Check's DC

Group Initiative or Popcorn Initiave

Player Facing Combat: Monsters have an attack bonus (Atk +12) and instead of rolling the attack, the player rolls for defense (their AC - 10 is their AC mod and they roll a d20). So instead of a Goblin having a +3 to attack, they have an attack of 15, and if the player has an AC of 16 normally their AC mod is 6, and they roll a d20, add 6, and if that meets or beats a 15, they successfully defend.

The Escalation Die. This is from 13th Age. D6 that adds +1 each turn until maxed, and adds the number to all attacks.

Flashbacks. From Blades in the Dark. Maybe a Hit Die cost to engage in a flashback. It's not time travel, but it can retroactively allow them to do something. It helps to eliminate the planning phase.

Gritty Realism or Gritty Health. Long rests cost a week, and short rests a day. This is just too difficult for most games.

Vitality Points. Too much extra math.

Spell Points. Too much extra math.