Ok so in eldest when Oromis is telling eragon about the history of palancar valley he tells of kind palancar who was the first human ruler in agalainsia. Then about how he took and fortified Palancar Valley and attacked the elves and was defeated. Later on on the king was assainated by one of his heirs and his bloodline is still present in Palancar Valley the place where all humans in agalainsia came from. My theory is that all riders in some way are an heir to king Palancar as the first ever rider eragon was also from palancar valley meaning his blood is recognised by the dragons which means that the bloodline of the dragon riders could all be descendant of kind palancar.

Any comments or other theory's as to what makes a dragon hatch for a rider.

If roran Is a heir to the throne then what about Eragon?

well... roran is gonna be king...
but Eragon's a rider and, with Galby fresh in their minds, i doubt that the ppl of alagaesia will let another rider be king.

and roran is human. so it makes sense.

the original Dragonrider, Eragon, was an elf, so I'm pretty sure that he wasn't from Palancar valley. so that would mean only the elf rider's could be decedents. and remember that there were riders along time before the humans showed up in Alagaesia.

there were? o ya! lol

yeah but Eragon is going to the elves homeland never to return

also i do think roran is going to be the next king

it doesn't say anything about Eragon going to the Elves homeland in any of the books

it doesn't but it makes the most sense

Well Eragon wouldn't be the heir because Roran is the oldest.

And Eragon might not go to the elves homeland, we have no idea what is out there at all

that would be cool if he did though.

mabye eragon will sail the sea

they say he will fly across the sea