Homebrew Rules

Climb onto a Bigger Creature
If one creature wants to jump onto another creature, it can do so by grappling. A Small or Medium creature has little chance of making a successful grapple against a Huge or Gargantuan creature, however, unless magic has granted the grappler supernatural might.
As an alternative, a suitably large opponent can be treated as terrain for the purpose of jumping onto its back or clinging to a limb. After making any ability checks necessary to get into position and onto the larger creature, the smaller creature uses its action to make a Strength (Athletics) or Dexterity (Acrobatics) check contested by the target’s Dexterity (Acrobatics) check. If it wins the contest, the smaller creature successfully moves into the target creature’s space and clings to its body. While in the target’s space, the smaller creature moves with the target and has advantage on attack rolls against it.
The smaller creature can move around within the larger creature’s space, treating the space as difficult terrain. The larger creature’s ability to attack the smaller creature depends on the smaller creature’s location, and is left to your discretion. The larger creature can dislodge the smaller creature as an action—knocking it off, scraping it against a wall, or grabbing and throwing it—by making a Strength (Athletics) check contested by the smaller creature’s Strength (Athletics) or Dexterity (Acrobatics) check. The smaller creature chooses which ability to use.
Overrun When a creature tries to move through a hostile creature’s space, the mover can try to force its way through by overrunning the hostile creature. As an action or a bonus action, the mover makes a Strength (Athletics) check contested by the hostile creature’s Strength (Athletics) check. The creature attempting the overrun has advantage on this check if it is larger than the hostile creature, or disadvantage if it is smaller. If the mover wins the contest, it can move through the hostile creature’s space once this turn.
Hitting Cover When a ranged attack misses a target that has cover, you can use this optional rule to determine whether the cover was struck by the attack.
First, determine whether the attack roll would have hit the protected target without the cover. If the attack roll falls within a range low enough to miss the target but high enough to strike the target if there had been no cover, the object used for cover is struck. If a creature is providing cover for the missed creature and the attack roll exceeds the AC of the covering creature, the covering creature is hit.
Cleaving through Creatures If your player characters regularly fight hordes of lower-level monsters, consider using this optional rule to help speed up such fights.
When a melee attack reduces an undamaged creature to 0 hit points, any excess damage from that attack might carry over to another creature nearby. The attacker targets another creature within reach and, if the original attack roll can hit it, applies any remaining damage to it. If that creature was undamaged and is likewise reduced to 0 hit points, repeat this process, carrying over the remaining damage until there are no valid targets, or until the damage carried over fails to reduce an undamaged creature to 0 hit points.
Healing Surges This optional rule allows characters to heal up in the thick of combat and works well for parties that feature few or no characters with healing magic, or for campaigns in which magical healing is rare.
As an action, a character can use a healing surge and spend up to half his or her Hit Dice. For each Hit Die spent in this way, the player rolls the die and adds the character’s Constitution modifier. The character regains hit points equal to the total. The player can decide to spend an additional Hit Die after each roll.
A character who uses a healing surge can’t do so again until he or she finishes a short or long rest.
Under this optional rule, a character regains all spent Hit Dice at the end of a long rest. With a short rest, a character regains Hit Dice equal to his or her level divided by four (minimum of one die).
For a more superheroic feel, you can let a character use a healing surge as a bonus action, rather than as an action.
Exhausted. While you are subjected to the Exhausted Condition (known in older books as Exhaustion), you experience the following effects:

Levels of Exhaustion. This Condition is cumulative. Each time you receive it, you gain1 level of exhaustion. You die if your exhaustion level exceeds 10.

d20 Rolls Affected. When you make a d20 Test,you subtract your exhaustion level from thed20 roll.

Spell Save DCs Affected. Subtract your exhaustion level from the Spell save DC of any Spell you cast. Ending the Condition. Finishing a Long Rest removes 1 of your levels of exhaustion. When your exhaustion level reaches 0, you are no longer Exhausted.
Grappled. While you are Grappled, you experience the following effects:
Speed 0. Your Speed is 0 and can’t change.

Attacks Affected. You have Disadvantage on Attack Rolls against any target other than the grappler.
Movable. The grappler can drag or carry you, but the grappler suffers the Slowed Condition while moving, unless you are Tiny or two or more Sizes smaller than the grappler.
Escape. While Grappled, you can make a Dexterity or Strength Saving Throw against the grapple’s escape DC at the end of each of your turns, ending the Condition on yourself on a success. The Condition also ends if the grappler is Incapacitated or if something moves you outside the grapple’s range without using your Speed.   Hide. With the Hide Action, you try to conceal yourself.
To do so, you must make a DC 15 Dexterity Check (Stealth) while you’re Heavily Obscured or behind Three-Quarters Cover or Total Cover, and you must be out of any visible enemy’s line of sight; if you can see a creature, you can discern whether it can see you.
On a successful check, you are Hidden. Make note of your check’s total, which becomes the DC for a creature to find you with a Wisdom Check (Perception).
UNARMED STRIKE An Unarmed Strike is a melee attack that involves you using your body to damage, grapple, or shove a target within your Reach.
Your bonus to hit with an Unarmed Strike equals your Strength modifier plus your Proficiency Bonus. On a hit, your Unarmed Strike causes one of the following effects of your choice:
Damage. The target takes Bludgeoning Damage equal to 1 + your Strength modifier.
Grapple. The target is Grappled, and the grapple’s escape DC equals 8 + your Strength modifier + your Proficiency Bonus. This grapple is possible only if the target is no more than one Size larger than you and if you have a hand free to grab the target.
Shove. You either push the target 5 feet away or knock the target Prone. This shove is possible only if the target is no more than one Size larger than you.