I don't think Corrin and Euden would have a lot of overlap. I know the dragon transformation gives them an immediate comparison, but the way the two games and characters use it is pretty different. What makes Corrin is that they use their
Dragon Fang: a trait that allows them to shapeshift limbs into claws, spears, projectiles, and so on. That ability makes up the lion share of their moveset and is what gives the character their flare, being able to turn into a dragon is ironically the least used aspect of Corrin as offhand, they only have three moves that properly use it along with the Final Smash. By comparison, Euden would have to full shapeshift to make full use of his dragon as that's how the mechanics of DL works, so that aspect is immediately lost as there would ideally be more emphasis on the dragon.
It's especially is the case when it comes to their dragon form. The one that would be most similar to Corrin's isn't the most marketed or popular so she likely wouldn't have much (if any) influence, while the one they would most likely use is nothing like Corrin's dragon form. They don't share elementals, body types, abilities, or otherwise so they would be very distinct from each other. If anything their dragon forms would be the biggest differential, not what makes them too similar
thinking about it, just make him playable.
I do agree his base form is the biggest flaw to him since if they use that there's more of an apparent overlap with Roy. Gala would have to be his saving grace there, and admittedly I'm not sure they'd go for it.