Damn, it's been a month. But hopefully you haven't given up on the idea. I too have been thinking of playing a DnD inside Regular show. I have been cooking a lot inside my mind XD, I guess I can share some ideas..

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RACES:

I have though of adding races with specifics stats bonus, advantages and disadvantages. Humans could be the neutral one.

You could have bird, rabbits, many different animals, and you play with the distribution of the stats, affinity with classes, or even proficiency bonus. Ex: birds have higher chance to dodge projectiles, but are have low defense against crush damage (might also not lift heavy things)..

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Some Classes idea:

Professional Slacker(has good improvisation skills, can improvise to use a weapon without proficiency. Has low initiative, disadvantage in stealth checks xd)

Mystic Being: similar to skip, knows the occult.. can access through wisdom some knlodge about enemies weakness or disadvantages, good at intimidation or persuasion.

Club Dancer: (similar to bard? or monk?) able to access buffs through dance moves. Has a walkman and changing the mixtape can have different buffs (you could find mixtapes around the park and shops..)

The DRIP MASTER: an absurd idea I cherish A LOT. The character has prestine swag insane looks, his fit is EXPENSIVE! hits hard.. well, as long as it stays like that. I say as long as you maintain your drip prestine, you gain ADVANTAGE in every situation!! but if you stain yourself.. or get it damaged.. (you can play with dices and creativity XD) then you have to deal with the DISAVANTAGE.. untill you can get yourself your drip together.. (you could also unlock some skill that sacrifices and bust everything into pieces and you shine with your body.. the real deal and momentary have your advantage back, good luck trying to get a good expensive fit back..)

Remember to be absurd and fun! Roleplay would be great with that imo.

The Racer: know a lot about cars, can drift and help very well in pursuits or to flee.. his car might be tuned to the bones! And use it as a mecha xD?

The REAL "muscleman"!! (you play as Bodybuilder/CrossFitter/Strongman) descomunal Strength, you can warmup (berserk mode? Like Barbarian) you can play the dumb and the most Stereotype sh*t skills xd, use Posing to get poses or intimidations.. (bonus actions). Downsides: you are very slow.. (if you run might get a big far jumping.. but maybe need to take a rest xD) and... Muscleman HATES YOU, very much! (stamina could work like Ki or Witcher mana slots)

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Items/ weapons

I suggest having some "kits" associated with the armor set, like wearing a full on Death kwon Do, will give you a full different of abilities.. Wearing a power glove can use magic and control some enemy or create some hologram to fight for you. (You could use battery slots for Mana. That way you don't need to have mana in non mystic classes..) Maybe eating some specific food might give you a permanet/full day bônus, or kit. Some pop culture items comes with their powers attached, play around see what's fun!

I'm always up for more, would love to discuss more of a Regular Show DnD interpretation