Neltharion having earth powers is an interesting observation but there’s actually already an earth Primalist. If you look up Iridikron’s model on wowhead, he’s made of stone:
https://www.wowhead.com/npc=185407/iridikron
And this is a side tangent but they’re not all from the same clutch, Ysera and Alex are the only ones biologically related. Alex said she and the Primalists were ‘as close as clutchmates’, meaning not actually related but such good friends they basically were.
I definitely think we’re going to learn more about the origins of dragons and see more of Neltharion’s legacy but I don’t think this is it, honestly.
I know of at least one questline that has unique dialogue for dracthyr (minor spoilers, it’s a sidequest not main story) (Spoilers) DF sidequest: The Black Locus