Seething Overthink

At 3rd level, you can invoke Seething Overthink, {proficiency bonus} times per day. An opponent of your choice must make a Wis saving throw or be compelled to devise an overcomplicated attack on their next turn, instead of whatever they were going to do. Whatever it is must require an Acrobatic or Athletics, or tools check, whichever is worst for them; if they fail, it's the equivalent of a critical fail and they are incapacitated for the next turn. If they succeed, then they roll an attack as normal.  

signature weapon

Also at level 3, the Blissfully Stupid barbarian can adopt a signature weapon, in the form of a mundane non-weapon object. When you attune yourself to this object (which requires giving it a name), you can cause it to act exactly like the weapon of your choice—for instance, a daffodil that acts as a handax, called the Daffyax. The signature weapon has no other special properties, except that the first time an opponent sees it in action, they must do a Wisdom check against the barbarian's Stupidity DC or be surprised.   Nobody else can wield the barbarian's signature weapon. Only one signature weapon can exist at a time; once you've created one, you can't create another until you destroy the current one.  

Relentless Coincidence

At 6th level, you can invoke Relentless Coincidence for yourself or another target, also {proficiency bonus} times per day. For the next 10 minutes, the target can subtract d3 from opponent's successful attack rolls. If the result means a miss, it's because of some weird banana-peel-level accident.  

semi-beast form

At level 10, the Blissfully Stupid barbarian can transform themselves at will into a hybrid beast form, part-human and part-animal, choosing one of the types below. The beast must be at least one size larger or smaller than the barbarian. Your size changes when you transform; you remain bipedal and generally humanoid, but gain the secondary features of your beast form (fur, feathers, horns, etc). You gain the movement, stealth, and/or AC traits of your new size, plus some traits per type:   Options for beast form:

• Moose, elk, or reindeer (large): You can make a headbutting attack; If successful, you do 1d8+Str bludgeoning damage, and the opponent must make an Acrobatics check against your Stupidity DC or be knocked prone. Every time you do this, do a Con check against DC 12; if you fail, you now have a killer headache that causes you to attack with disadvantage. Your headache goes away between combat encounters.
• Sheepdog (can be small or large): You can use your movement to herd a number of opponents, up to your proficiency bonus. Each opponent must make a Wis check against your Stupidity DC, or move with you. In combat, they can make this check once per round.
• Chicken or goose (tiny or small): You have the self-perceived invulnerability of medium-sized barnyard fowl. Once per day, you can ignore the entire damage from an attack as though it didn't happen.
• Opossum (tiny or small): As a bonus action, make an awful screech that frightens anyone who fails a Con check against your Stupidity DC
• Pygmy bluetongue (or other tiny lizard): Make stealth checks with advantage. Also, if you remain unmoving for at least thirty seconds, a successful stealth check means other creatures in the area will forget you're there. When you move again they will be surprised.
• Sloth, capybara, or wombat (can be small or large): Movement is halved. At the beginning of combat, each opponent must make a Wis check against your Stupidity DC; if failed, they attack at disadvantage for the rest of combat, because you seem so nice.

  You can remain in your hybrid form as long as you want, but sleeping or losing consciousness causes you to shift back.  

Contagious Stupidity

At level 13, you can invoke Contagious Stupidity {proficiency bonus} times per day. You can attempt to convince anyone in earshot of anything at all—that they like you, that the moon is a stage prop, etc.; if your Persuasion check succeeds, they will believe you, and act as such for the next 10 minutes. Other un-Persuaded people can snap them out of it with a successful Persuasion check against your Stupidity DC.