I think that Dylan & Arthur would probably be one of the stronger relationships in the weird web that is my idea of the UK polycule. Barely coherent headcanon rambling below.
It's a relationship that started with abuse from when Dylan hexed the ever-loving fuck out of Arthur in their youth, and followed swiftly by Dylan getting fucking conquered. So now they're forced to be together, and... unlike being resentful and contrarian like Alistair, Dylan rolls with it. Gets to know Arthur. Makes up for his previous shortcomings by turning into a model older brother. After he's conquered, Connor and Alistair view him with warm memories, but he's "too far gone." A disconnect that still kind of remains. Meanwhile Dylan still loves them as much as he used to, but he's much closer to Arthur -- even though Arthur is technically his owner. Definitely some elements of Stockholm syndrome. But the thought of him redeeming himself over the course of centuries and becoming someone Arthur can really rely on and confide in, is just so fucking good. With angst for seasoning -- Dylan still feels terribly guilty about abusing him as a kid, and is insecure about Arthur relying on others more than him. Equal parts jealous and worried that he isn't doing a good enough job. Arthur, meanwhile, feels comfortable enough around Dylan to not hold back on some of his more unhinged thoughts, and when he isn't well, or having a meltdown, he'll say or do the most heinous shit and expect Dylan to be in his corner, because Dylan is always in his corner. Which is hard to navigate, because it takes a lot to talk a narcissist like Arthur down from something without making him feel completely betrayed. But after centuries of practice, Dylan knows how to do it... Most of the time. +90% of their relationship is good and functional, and they get a lot of comfort and support from each other. So the fluff potential is off the charts, too.
There's so many facets to the relationship that it can work with pretty much any view of their sexual dynamics -- I'm a big proponent of versatile Arthur rights so naturally I can think of 1,000 different scenarios. 🥺 God I wish I was better at anatomy, they'd be fun to draw. Not that I could post the good shit here. 🙄