The first story of Paperinik (Donald Duck's superhero-antihero alter ego) shows he was inspired by the journal of Fantomius, a Gentleman Thief who is also stated to have died years before the story, and the second story introduces the figure of Dolly Paprika, Fantomius' lover. Fans have interrogated themselves on their story for years, until they got a series about their adventures... But where did they disappear to still remains a mystery.
What happened to Della Duck? And who is her husband (and Huey, Dewey and Louie's father)? A dutch story gives a possible answer to the former, but the latter remains a mystery.
Goofy's family. In one story it's even hinted he's related to aliens (something that Mickey notes would explain a lot of things).
The origins of the Super Goof's power-granting peanuts.
Ensemble Darkhorse: The very reason for the Duck universe's existence! And then it got another Darkhorse of its own when Scrooge McDuck came along...
They Wasted a Perfectly Good Line Art: The vast majority of the comics, at least in the United States. Characters will change the color of their shirts between panels, entire characters will be rendered in one color, gradients are used like they're going out of style, and very little background art is colored with detail. This is especially prevalent in Don Rosa comics, where the intricately detailed artwork is half the appeal.