The Frontier

Custom DND rules

Action: Use for an ability check, skill check, attack, cast a spell or use a special ability
Bonus action: Use for specific abilities classified as bonus action
Reaction: Use a reaction to perform actions outside of your turn
Movement: Use your movement speed to move around the map

Step: If you are engaged in melee you can step 5ft without triggering an opportunity attack
Raise Shield: Use an action to raise your shield to double it's AC bonus
Hunker Down: When in cover you can improve the cover factor by 1 e.g. a 1/4 cover can improve to 1/2 cover
Take Aim: When trained with crossbows or firearms, use an action to end your turn and focus on aiming. At the start of your next turn to fire with double your attack bonus. You can also use your reaction to fire with your normal attack bonus.
Rapid Shot: When you haven't moved you can sacrifice your movement to use your bonus action to make an extra ranged or thrown attack with disadvantage
Focused attack: When you haven't moved you can sacrifice your movement to use your bonus action to make an extra melee attack with disadvantage
Focused magic: When you haven't moved you can sacrifice your movement to use your bonus action to to cast a cantrip. If the cantrip uses your spell attack bonus you roll with disadvantage and if it has a saving throw the target rolls with advantage

Character building:

Level up by milestones
Fixed hitpoints average
Abilities: standard array or roll (4d6 per ability score and take the 3 highest die for the result. You may take one reroll.)
Discuss with the DM if you want to customize your character
Optional rules are allowed
You may spend up to 150gp on equipment and keep the difference on your character sheet

The below applies to both players, NPCs and antagonists.

Once we've started the session no talking out of character unless you have a question for the DM
Nat 20 is always a success and special things may happen
Nat 1 is always a failure regardless of feats, modifiers, etc. You may use features, feats or conditions to reroll. Bad things may happen.
When you exceed a check by 10 points it becomes a critical success and when you have less then 10 points it becomes a critical failure.
Outside of combat all healing potions and spells are maxed
Use of the Help action for other players only if you can explain how your character helps
Luck points may be donated to other players if you can see each-other
You may roll a Sleight of Hand check to cast spells covertly. DC goes up by spell level (DC10 + spell level; cantrips are spell level 0) and situation.
Players start the campaign with 3 Group Inspiration dice which can be used to reroll any dice (skill checks, saving throws, damage, etc.). More dice can be earned by being awesome!
Personal inspiration can be earned by doing cool stuff, impressive narration, funny jokes and bribes to the DM :-)