Well. There are so many things about Ash and Pikachu that come to mind when talking about their characters, Pikachu is in every way Ash's best friend, he is the one who spends the most time with Ash and knows him more than no one else and the opposite is also important to be shown, I feel like people see what Pikachu does for Ash but don't see what Ash also does for Pikachu (aside from the saving Pikachu life thing), about how they are individually, about how Ash listens to everything Pikachu has to say, about how Ash cares about how Pikachu feels, about how he cares about the little things in his day I believe that many series don't convey this much outside of the original series, Shudo was great at that...me AG Ash and Pikachu are perfect, Ash's Pokemon interact with Pikachu and each other, the team's Pokemon interact with May's Pikachu and Ash, sometimes the characters ask Pikachu or talk to Pikachu when he isn't there Ash's shoulder or when he is..., DPAsh and Pikachu are an inseparable and unstoppable duo ✨😭 I love how everyone talks about Pikachu and Ash but at the same time Pikachu being his own self, people interested in Pikachu as a character (like Paul, despite not respecting Ash, he had great respect for Pikachu's power to the point of staying in Ash's gym battle because of Pikachu and Pikachu and Ash bound's determination)
I hope this helps and gives you some inspiration.
About the Headcanon part...I don't know Maybe I'd like to hear more about Headcanon from others, I don't know much about the fandom in general, I like creating my own stuff.
But I would love to see more Headcanon about Ash and Pikachu