Or, Marco and Thatch get captured and Marco nearly goes insane, along with his cellmate. Not canon-compliant, but (hopefully) quite amusing. Based on the truth serum scene in Ant-Man and the Wasp, but with two people doing it.
A plot bunny I got off an author note somewhere.

Marco groaned. He was tied up in kairoseki chains, somewhere in the expanses of a massive prison. But that wasn't the worst part of it. He had been thrown in a 3-person cell with none other than his brother, and 4th division commander, Thatch. And therein lied the problem. Thatch with the entirety of the Moby Dick to play with was one thing, but Thatch, tied up and with no way to vent his boredom was a whole other universe away from the original thing.

"Please can you just shut up -yoi!" He said exasperatedly, "The entire prison can probably hear your 'heroic exploits' by now!"

"Ooh, ooh, wouldn't that be cool though, me broadcasting a radio show or something via den den mushis! I could call it... Thatch broadcasts! And I'd be famous, and there would be loads of lovely women, just queueing up to meet me! Little young me! Perhaps then I could open my own restaurant or something... but wait, that would be disrespectful to Pops... ah! I could name it after him! And still be fourth division commander! After all, my division is pretty much all chefs! It would be the best restaurant in the world!" And on and on and on. Thatch was like a walking tone dial!

"Oh, mi madre used to have a collection of tone dials in her restaurant, but they all played Morrisey. It was like a shrine or something. And whenever someone disagreed, she would be all like, 'you no like Morrisey? Then get out of my restaurant!' Strangely enough, no-one ever disagreed. Her cooking was too good to disagree with!" Oh damn, he hadn't realised he'd said that out loud. How long had he been doing it for?

"Not too long, actually. I'd say about... 10 minutes or so. What's a tone dial anyway?" Oh right, he'd forgotten there was someone else in here, he'd been too busy being driven insane by his brother. And of course the rookie didn't know about tone dials, they weren't commonly available this side of Skypiea. Or Jaya for that matter. The connecting islands were further on as well. He kinda wished he could go back to Skypiea, it had been 10 or more years since he'd been. Ah, how he missed the feeling of soft, island cloud on his feet. Plus, just bringing a sack of dirt could probably buy him most of the dials in the place. Where did the dials come from anyway. Oh damn, he was starting to sound like Thatch. Speaking of which, where did Thatch's mum get those tone dials anyway? And why Morrisey? Everyone knew the Mushis were better!

"Wow, I never knew you had so many interesting thoughts, Marco! I must agree, the Mushis were better, but mi madre just loved Morrisey. All of his songs were there, even the later ones. I think her favourite was 'war dog'. You know the one? 'You ain't nothing but a war dog, crying all the time, you ain't nothing but a war dog—"

"Yes I know the one -yoi. I don't see how anyone liked that one really. It was just the same lyrics, round and round and round. Too repetitive. How about modern music? I quite like Flyin' Man's The Phoenix, myself. Always have enjoyed a bit of heavy metal -yoi. Gets me through all of that damned paperwork!" Oh damn, he'd done it again. He hadn't wanted anyone to find out about that one, least of all Thatch. He'd probably steal his baby den den now!

"Wasn't going to, but thanks for the idea!" Oh shit. "And besides, that one is practically named after you! I like a bit of Ned Shepherd, especially his new one... what was it called? Perfect, I think. But then, I always have been a hopeless romantic for things like that. Hey, guy in the corner, how about you?" Ah, yes. The guy in the corner. Never did answer his earlier question. If he just thought it would the guy hear?

"Yes, I would. And the name is Ace, not 'guy in the corner'. As for me, I quite like ASL's 'I'm not famous'. I find it rather ironic."

"Ace? As in Portgas D. 'Firefist' Ace? Oh that's so cool! Pops has wanted you to be a part of our family for ages! Ever since your dismissal of the World Government! I must say I agree. You seem like a cool guy. And I bet your crew could join too! Hey, if you want, I could tell you stuff about us, see if you want to join. Though, once Pops has set his eyes on you, try all you like but you'll join eventually! So how about it?" Thatch asked.

Marco didn't get to hear the reply, as the piece of wall he was attached to disappeared, and he moved backwards through the wall. Well shit. What had he gotten himself into? Maybe he was being interrogated, or cut open! He didn't want to be cut open! Not when he couldn't heal!

"Well, look who we have here. Marco the Phoenix! Let's get some information out of you. My mind serum should be active now. Let's begin, shall we?"

Another writer's block buster, brought to you by a sufferer!
All songs and artists are based off of real ones, and quite a lot of rambling was in fact me writing down my thought process.
Sorry about the ending, I needed a way for Marco & co (punny) to learn about the serum!
I will try and update regularly... but try is the main word here.
Wish me luck!

Reviews, faves and follows always appreciated!
Try and guess songs and artists! If anyone has any topics they want to see brought up by... whatever this is, then review or pm me!