Most of these appear in the novella-length fanfic I haven't uploaded anywhere yet, but for now here's a list of some of my own ideas about Warcraft's dragons.
Sartharion was Onyxia's prime consort in addition to being the keeper of the Obsidian Sanctum. (This helps explain why he was such a big deal, since in-game we never got any explanation as to who he was or why we should care.)
The Sanctums under Wyrmrest Temple served as a safe place for any dragon from that flight to lay their eggs, if they didn't have a lair of their own. Orphaned whelps could also seek shelter there if they had no where else to go.
"Samia Inkling" is actually Sabellian's mate, and the whelp that flies messages back and forth between them in Blade's Edge is their son. (Alternatively, other fanfic writers have made her his daughter, or a dragonsworn human.)
The dragons who fly over the high plateaus in Blade's Edge (Insidion, Furywing, Rivendark and Obsidia) are two mated pairs. They started laying their eggs up there because the wyrmcultists kept stealing them.
Proximity to the Apexis crystals in Blade's Edge is partially responsible for cleansing some of the Old Gods' corruption from the black dragons there.
Obsidia is the younger sister of Nefarian, Onyxia and Sabellian, and is the daughter of Neltharion and Sintharia.
The Old Gods sometimes scramble the brains of black whelps before they hatch, and the resulting hatchlings are either uncontrollably violent and have to be put down, or they're so simple-minded that they cannot speak or shapeshift, almost reverting to the mentality of their proto-drake ancestors.
Breeding between different colors of dragons is rare due to social taboos (which is canon from Night of the Dragon) but when it does occur the eggs rarely hatch. The greatest success rate in cross-flight breeding is when the mother is a red dragon, because of their powers over life.
The colors of proto-dragons who did not get an Aspect to lead them did not completely die out, but inter-bred with the five main flights. Purple and orange females mated with Neltharion, explaining the coloration that has been passed down to Onyxia and Nefarian. White dragons interbred with blue, and sometimes their genetic material resurfaces, resulting in the coloring seen in the "albino drake" mount model.
Chromie/Chronormu is a biologically male dragon who identifies as a female, thus preferring female mortal forms. (I know this is a headcanon I share with a lot of other people, but I like it.)
Deathwing was directly under the Old Gods' control after the battle at Grim Batol, when he fled to Deepholm. His will was completely overridden by the Old Gods, explaining why none of his brilliant, Machiavellian scheming appeared during Cataclysm.
Fahrad's real name is Fahradion, and he's related to Searinox, the black dragon who appears in one of Arthas' campaigns in WC3 in the Alterac area.
Nyxondra's original mate was among those trapped in Outland when the Dark Portal closed. Fahrad later hooked up with her as well. (Sometimes I think he's Wrathion's real father and other times I don't. My fanfic is obviously leaning in that direction.)
Kalaran is Nyxondra's younger brother.
The three drake bosses in Blackwing Lair and the three similarly-named drakes around the central lava pit in Blackwing Descent are all Nefarian's children.
[I'll add more later as they occur to me.]