The idea of all these is to work off the existing system to give some additional options, and a nod to versimilitude, without affecting regular gameplay. I'm pretty confident of all of them except maybe the Grappled Casting rule, which is designed more for lower-magic settings.
Campaign House Rules - Combat
The following incur opportunity attacks if done within enemy reach:
Shooting a bow, except a crossbow
Reloading a bow, gun or similar weapon
Standing from prone
Picking up an object (such as a dropped weapon) from the floor and standing back up
The following use up half a character's movement:
Standing from prone (as per RAW)
Picking up an object from the floor and standing back up
Mounting or dismounting a mount.
Grappled Casting: A character who attempts to cast a spell while Grappled must make a CON save at DC 10 or else lose the spell. If Restrained or Grappled & Prone, the save is with Disadvantage.
Mobbing: When multiple attackers of the same size category surround their opponent (ie are within 5'), the 5th+ attackers that round have Advantage on their attacks.
Disarming: Attack is with Disadvantage (& can't attempt a disarm if already at disadvantage); if it hits then target takes damage & makes a STR or CON save (defender's choice) at higher of DC 10 or half the damage dealt to not drop object. Save is with Advantage if using both hands.
Campaign House Rules - Combat
The following incur opportunity attacks if done within enemy reach:
Shooting a bow, except a crossbow
Reloading a bow, gun or similar weapon
Standing from prone
Picking up an object (such as a dropped weapon) from the floor and standing back up
The following use up half a character's movement:
Standing from prone (as per RAW)
Picking up an object from the floor and standing back up
Mounting or dismounting a mount.
Grappled Casting: A character who attempts to cast a spell while Grappled must make a CON save at DC 10 or else lose the spell. If Restrained or Grappled & Prone, the save is with Disadvantage.
Mobbing: When multiple attackers of the same size category surround their opponent (ie are within 5'), the 5th+ attackers that round have Advantage on their attacks.
Disarming: Attack is with Disadvantage (& can't attempt a disarm if already at disadvantage); if it hits then target takes damage & makes a STR or CON save (defender's choice) at higher of DC 10 or half the damage dealt to not drop object. Save is with Advantage if using both hands.