Inspiration

You can spend inspiration to re-roll any dice roll after the roll has been made. This is not limited to d20 rolls. Pools of dice for damage rolls or random tables are also eligible.

Potions

Drinking a potion is a bonus action. Administering a potion to an unconscious character requires an action.

Flanking

Melee attackers have a +1 bonus to hit for each of their allies holding melee weapons threatening the space of their target.

Dropping to 0

When a character drops to 0 hitpoints they gain 1 level of exhaustion.

Spears

Spears have reach.

Magic Weapons

Magic weapons simply deal their normal damage type (bludgeoning, piercing, or slashing). Most monsters that would have resistance, or immunity to non-magic weapons do not, although they may still have resistance to one of the normal weapon damage types if it makes sense.

Legendary Resistance

Monsters do not have Legendary Resistance. Instead they have 1 extra use of their Legendary Actions per round, and one new ability to spend their Legendary Actions on:

Resist. Spend 1 Legendary Action as a reaction to re-roll any failed saving throw. You may use this ability more than once on the same saving throw, but must spend another Legendary Action each time.

Bonus Proficiencies

Characters gain a number of extra skill / tool proficiencies, or languages equal to their Intelligence modifier (minimum of 0).

Sprint

At the beginning of a character's turn they can declare that they are sprinting. When they do they can do nothing except move that turn and can move up to four times their speed. Attacks of opportunity made against them that turn are made with advantage.

Rolling for Hitpoints

When a character levels up they roll a hitdice to determine how many hitpoints they recieve (adding their Constitution modifier as normal). If they do not like the result of their roll they can choose to re-roll, however the new roll is made with a dice of 1 value lower than the previous. They can repeat this multiple times if they continue to not like the result of their roll. The minimum dice value that can be rolled is a d4, so you can not re-roll after rolling a d4. Dice values in order are: d12 - d10 - d8 - d6 - d4.

For example if a character takes a level in Barbarian, they roll a d12 to determine how many new hitpoints they recieve. If they do not like the result of that roll they can re-roll using a d10. They can take that result or re-roll again, using a d8, and so on.