Flanking allows both your character when working with your allies, as well as groups of your enemies, a simple way to gain advantage when making attack rolls against a common foe.
A creature can't flank an enemy that it can't see. A creature also can't flank while it is incapacitated. A Large or larger creature is flanking as long as at least one square or hex of its space qualifies for flanking.
When a creature and at least one of its allies are adjacent to an enemy and on opposite sides or corners of the enemy's space, they flank that enemy, and each of them has advantage on melee attack rolls against that enemy.
DMG p251
When choosing to roll for hit points during character advancement a player may re-roll ones when rolling for those hit-points.
Drop the ‘normal’ inspiration rules, players get a “charge” of inspiration at the beginning of each session. Still a binary on / off mechanic.
A character can drink a potion, themselves, as a bonus action. It remains a full action to feed a potion to another creature.
Using Bardic Inspiration as an example; many abilities, features, feats, etc. have the following or similar verbiage:
The creature can wait until after it rolls The D20 before deciding to use the Bardic Inspiration die, but must decide before the DM says whether the roll succeeds or fails.
In any and all of these cases, this game will allow more leeway to the timing of such abilities, it will be possible to use these abilities after the result of a role has been declared by the GM, within reason.