I'm working on a barbarian that only speaks one word with various gestures to communicate. He's actually very smart, and is from a clan that is so smart their language only requires the one word various gestures and inflections to convey every possible meaning. He understands and does in fact speak most other common tongues, but chooses not to out of disdain for the fact that nobody will learn to speak his. 

I've got tons of ideas like this that I'm writing up right now, but this one that could make a fun NPC I figured I would toss your way. 

If you're looking for something a little more mature. There is the multiple personality(just 2) female cleric/warlock. Switches between the personalities at inopportune times. For example, praying the in church as the cleric(which is a nun), and switches to the warlock(who is very promiscuous). Gods damnit she thinks. Not again. There are a lot of cute choir boys in this church, and the priest is kind of a looker himself though.... so maybe this isn't all bad. By contrast the Nun takes over and wakes up on the seedy side of town underneath a dwarf, and... oh my god is that half orc underneath me. 

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I have to say that sounds like some good ideas i'm going to pick the barbarian one though because the players in my campaign have been getting out of control and trying to break out the prisoners in town every night. Your barbarian will confuse them for some time i'm thinking. That will give me enough time to make up a side quest so they can get back to goblin slashing instead of prison breaking. Though now that i think of it the the  2 personality Cleric/War lock would be a great NPC for battles. I mean Cleric's and War locks both have different skill sets. Imagine a grave domain Cleric Destroying a hoard of undead on one side but on the other side the PC's are getting demolished by a night hag. Well then once the grave domain cleric is done he switches to a war lock Hex blade and beats up the night hag.

My thoughts exactly on the Cleric/Warlock multi personality character. I was thinking that they would have access to all their spells as normal, but depending on what perosnality is in control at the time would have penalties to the rolls for using skills/spells of the other class. It makes sense. The warlock can use the healing spells, but doesn't really like using the goody goody cleric spells. 

I'm going to write it up as an actual character to play as well if you guys have an opening and are interested. I did just get signed on to a few campaigns already with a few of my more fun ideas.

There's the Ruckus the dwarf that thinks he's an Elf with Elvitiligo(a very rare life-threatening condition that causes him to look more and more like dwarf as he ages). He also has Tourette syndrom, but only blurts profanities in Elvish(becuase he's an Elf damnit). He's also very racist towards dwarves. - Signed on to Pirate Campaign

I just got Thelonious Gruvspinner  Half Elf - Junky - Bard/Rogue with a sentient Lute named Jone's that mostly makes snarky discouraging comments to him. Thelonious has it coming. Trust me. I wrote up not just a backstory on this one, but an actual little(but very entertaining short story that goes along with it) - Signed on to Sundering Campaign

Both of these I've already signed onto campaigns, but I've posted their very entertaining back stories here: https://www.dndbeyond.com/forums/d-d-beyond-general/story-lore/9066-share-your-character-backstory?page=48

Welcome to use them as well, as they would both make very fun NPCs to have around. 

For something more serious and interesting check out my idea for this character Duo. He could also make a very interesting NPC: https://www.dndbeyond.com/forums/d-d-beyond-general/looking-for-players-groups/91460-paladin-warlock-pst-prefer-evenings-interesting

I'm actually looking to get Duo signed onto a campaign next as a player if you're interested.