Ash and a Beldum? That's not a pairing you see every day, which is exactly why the few fics that do tackle it have such a fascinating dynamic. Without the instant affection Pikachu gave him, Ash would have to work for every scrap of trust with a creature whose default is to literally not move. I love stories that lean into that—Ash trying to teach a Pokémon that battles through psychic power and evolution, not speed and agility. It flips his usual energetic coaching style on its head.
Some writers use Beldum's psychic link to explore Ash's internal state in a way the anime never could. Instead of just shouting his feelings, he might have to project calm, or Beldum might passively reflect his chaotic emotions back at him, causing confusion. The slow, deliberate path to Metang and then Metagross becomes this huge payoff, a testament to a bond forged through patience, not just adventure. It makes the eventual battle prowess feel earned in a completely different way.
Honestly, my favorite take was a crossover-ish fic where Beldum's analytical mind constantly tried to calculate the probability of Ash's insane plans working, which was always hilariously low, yet it followed him anyway. That's the core of it, I think—Ash convincing a logic-based being to believe in heart and instinct.