Concept: dragons and dwarves as different stages in the life of the same creature.
Explanation: firstly, I'm fully aware that a transformation of a dwarf to a dragon happened in myth to Fafnir. I'm pretty sure that's the only occasion, and in that story it's clearly about the cursed ring and not a natural thing that happens to dwarves. However, one could write a story where it's a bit different. Basis to the idea can be found with the affinity of both these species to gold - dragons hord it, dwarves forge and shape it. Also, dwarves have originally in Norse myth evolved from worms digging through Ymir's corpse, and another common word for a dragon is a great wyrm or worm. Is it a stretch? Yeah, probably. But it can work!
Besides, way too many stories nowadays have dragons capable of shapeshifting between human form and their true, draconic form. In Sanderson's latest Cosmere installment (spoilers for Isles of the Emberdark etc), dragons appear to start their lives in a human form as part of their staging. So why not take things to a more logical conclusion? (Aside from the fact I never fully understand the idea of a dual form in a world where there are nonhuman races outside of dragons. Like, can dragons choose what they look like when they shapeshift, or is it something built in? And if it's the latter, can they shapeshift into species other than humans? Do they have set forms in all species?
Anyway, I'm open for discussion and further knowledge.