This is a game where it's fine to be metagamey, and might actually help the developers of D&D if they see this.  Basically.  A person will try as best as they can to abuse the game mechanic of the one above, and then give a game mechanic of their own for someone else to abuse.  This can be anything in the rules, but not homebrew.  Unearthed arcana is okay, as long as it is made by WOTC.  Any number of levels of any classes are allowed, and up to 3 allies are allowed.  Example:  Person 1 Says Elf, Drow subrace.  Person 2 says, "The Drow High Magic feat allows for a drow player to cast Detect Magic at will, and Levitate and Dispel Magic at level 4."  I know that isn't abusing it that much, but I think that some creative people could easily abuse game mechanics.

I got one that's gonna be hard to abuse: true strike.  Let the games begin!

A sorcerer can use quicken spell to cast a spell as a bonus action and then also cast a cantrip, in this case True Strike and then on the next round get advantage on their attack.

Since the cantrip usually takes an action to cast other casters can't take advantage of it like a sorcerer with quicken spell can.

Quote from Brian_Avery >>

A sorcerer can use quicken spell to cast a spell as a bonus action and then also cast a cantrip, in this case True Strike and then on the next round get advantage on their attack.

Since the cantrip usually takes an action to cast other casters can't take advantage of it like a sorcerer with quicken spell can.

 Great!  Now name a game mechanic for someone else to abuse!

Since no one said anything, I say the Mizzium Apparatus.

It gives a wizard spells it can’t cast, and basically a free spell casting focus. It’s a glove apparently, so you can also hold things, so.....

Bladesinger casts haste, then bladesings. Next turn(s), casts Heat Metal on opponent wearing plate armor, smacks them with a quarter staff, and has a familiar breathing fire. Now the question is how will it stop the heat metal? No saves!

edit:not a glove, but armor with a strap. Still works.

Ok;Thunderclap.

wizards usually can't wear armor... there are exceptions though

didn't quite understand how this post works, didn't realize I was supposed to name something at the end of my post so will do so now

Rations (food and water, page 185... only need to eat 1 ration every 3 days?... well 3+his/her CON mod, so it's possible to be 8 days?)

Capture someone and starve them for 2 weeks. Dead. Hold Person, snare, I don’t know. 

Thunderclap.

9th level divine Sorcerer 6th level evocation wizard casts a quickened contagion on anything(slimy doom setting), then casts thunderclap.  The creature takes the damage no matter what, and is now stunned.  Continue to cast a cantrip that the creature has disadvantage on saves on, with a sickness that causes it to not be able to do anything.  Free damage.

skywrite

Ten words, huh? Hmm...

take a dragon that’s in the sky fighting you. Sky write Blind. Clouds form in his face and he is blinded.

Demon Armor

Two options.

1.  Be Warforged.  You ignore everything about the armor.  Then optimize as you wish.

2.  Take a level in fighter and then all monk.  Get a +3 shield.  Get cleric to cast shield of faith and wizard to cast haste.  Enjoy awesome unarmed strikes and insane Ac. (28 to be exact).

Forgot to say thing to abuse.  Do nystul's magic aura

No. The magic item [magic item]Demon Armor[magic Item]

ugh. Look it up. I suck at tool tips 

edit: i see what you were saying. Hm. Ok then.

whats the mechanic?

Enter an airport full of deitys who are very vulnerable to Devastation Orbs. Cast it to block magic scanning radars, and start it. Boom! Just killed a lot of deities.

the letter A.

(joking. Violin)

Get tons of Violins, cast a 9th level animate objects on them.  Yes your friends will hate you for hogging the entire game, but you win the fight.  Also it would be funny to see a bunch of violins beat up a lich or tarrasque.

The Help action.

Use help. Have a familiar use help. Have it keep using help. Free Eldridge blasts!

homebrew

I mean, everything that isn't by the books is hombrew.  So all of your adventures in D&D are homebrew.  Why would you abuse those.  IF you really wanted to, you could make anything you wanted, to destroy a tarrasque in -1 seconds, or to make a monster that makes a tarrasque look like a pet rabbit.  To answer your question, I give you infinity.  It is the only real thing people can make with homebrew.  Infinite creations, worlds, characters, and monsters.  That is the heart of D&D, not the rules, but the things the rules help set the guidelines for.

Path to the Grave (Grave Domain cleric channel divinity)

Cross class into bard. Use it on a something. Level 20 monk and Fighter, each with you casting haste. Do I have to spell it out?

Yoi know what? Minor Illusion...... but only as a sound with no focus

Make the sound of a person's heart beating really loud so the guards think it is in their head, and then easily sneak by them.

Do you want to keep going?  It's pretty late where I am.

Edit: Alright.  Glibness

Midnight. Yeah. This is fun.

Quote from Yharim >>

Get tons of Violins, cast a 9th level animate objects on them.  Yes your friends will hate you for hogging the entire game, but you win the fight.  Also it would be funny to see a bunch of violins beat up a lich or tarrasque.

The Help action.

 Tarrasques have feelings too!

Level 20 (for charisma stat increases), that manual that gives you charisma, boon that gives you charisma, you have + 7 for Charisma checks. Automatic success DC 22 or lower.

Magic Missile

A 17th level Evocation wizard casts an overchanneled magic missile at 9th level.  If their intelligence is at least 20, thats an unavoidable 99 force damage.  If intelligence is higher, add 11 damage for every modifier above +5.

A bag of holding.

Put a tiny air pipe through the top so creatures can breathe, then get your bat/raven familiar to drop the bag into the BBEG’s room before going through the other parts of the dungeon, you have max hp and surprise him.

Frog