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.