My idea always had to do with the relation between Dragons/Proto-Drakes/Dinosaurs/various reptile races of Azeroth.
I have two theories of how they may be related:
1: Dinosaurs/reptiles on Azeroth were created by the Titans when they arrived on the planet. From the dinosaurs basic genetic structure the Titans created the Proto-Drakes and then from the basic structure of the Proto-Drakes they increased their evolution and infused them with their cosmic/arcane infusing super-technology to create the original Dragon Aspects and birthed the original flights.
2: My other theory is related to something based on the lore of the Earthen/dwarves that is revealed in Uldaman. The hologram keeper at the end of the instances says that the Earthen were created from a cross section of Azeroth's planet crust as a base material. My theory is that the titans did not create dinosaurs/reptiles on Azeroth but that they instead had existed on the planet during the primal chaotic reign of the Old Gods and their various lifeforms ruled, such as the early insectoid races and things like krakens and other nasties, would make sense that a savage race like dinosaurs would and primitive reptilian creatures would exist on such a hostile world. When the titans arrived and went about making the basic structure for the Proto-Drakes I believe that they took the genetic composition of these reptiles they found on the planet to create them, such as how the Earthen were created from resources already on the planet.
Also something else I am probably wrong about, but I have a feeling that the Doomlords of the Legion could possibly be the demon-form version of the Mogu. Correct me if I am wrong, but from reading notes about the warlock quest chain in patch 5.2 there is suppose to be a part I believe where you talk to your Doomguard (I think it was Doom, maybe Fel) and you can ask about its origins before its race became demons and from what I remember it says something along the lines about how its race were blood hounds of the Titans to track down arcade energy across the universe. As we pretty much know, the Mogu are creations of the Titans who possessed a great understanding of the arcane during a time when the majority of it was controlled by the Highbourne, also the Mogu do sort of have a similar bulky build as a Doomlord and there is also the slight coincidence that the quest chain even includes this dialogue at all in an expansion that has a large centering on the Mogu empire.
Ok so technically I guess those are theories so here are some random things thrown out like others have done:
-Hobgoblins (mooks) are actually the original forms of the goblin race. After being exposed to Kaja'mite an evolutionary process took place that turned a portion of the race into the short/intelligent race we know of now.
-The Flame Walkers of the Firelands/primal Azeroth were what the old god N'Zoth based the forms of the naga on when he transformed the Highbourne after the destruction of the Well of Eternity.
-Ok so nobody take this the wrong way: the reason why there are so few dark skinned human characters/NPCs on Azeroth is because the skin pigmentation change only occurred in humans living in societies in regions like Tanaris like the Wastewater Bandits.
-The Formless/The Shapeless (forget the exact name), the weird blob humanoid creatures that we saw in early concept art for Cataclysm but never actually got in the game are not actually Old God creations like Blizzard first said. When they are discovered in the future they will be a genetic creation of the Titans that used the malleable lifeforms to sculpt the humanoid creatures that would inhabit Azeroth and other worlds.
-Goblins are an obviously consumerist society while gnomes who lived in Gnomergan before the trogg invasion had a very communist life-style, all working together towards a common goal and is the reason why in Gnomergan you see instead of houses they all have large rooms with bunk beds inside of them. This is one of the main reasons why the goblins and gnomes have had such opposing views and hatreds for one another.
-I believe the Undermine looks like a goblin made underground version of New York City with rusted out piece metal sky scrapers, fast food joints, pawn shops, used rocket and trike salesman, houses of sin and various mines leading down into the earth. Also when we finally get to Undermine we will get player housing in the form of a goblin apartment we can purchase.
-Far below Azeroth there is a giant Titan complex far bigger then anything we have ever seen yet and was the primary area where the original bio-construction of the planets lifeforms took place.
-Likewise on one of Azeroth's moons there is a hidden titan complex that was the main base of operations for the titans themselves during the war against the old gods and overseeing the original re-origination of the world. Also there is a chance that one of the Old Gods was somehow imprisoned on the 2nd moon of Azeroth.
-Yu'lon and the serpents of Pandaria were one of the early designs of the Dragon Flights by the Titans, however instead of making them into protectors of the world they were merely added as a species onto it but still manage things such as shape-shifting, arcane control and high levels of intelligence.
-The ninjas you fight for the Bloodsail quests in the Cape of Stranglethorn are actually a long lost pirate crew that landed on Pandaria long ago and learned their combat techniques from the Shado-Pan. They have now returned to the South Seas to fight against their former rivals the Bloodsails.
-All of the original titan creations are actually technically a form of robots, even if they are statue like instead of like the Mecha-Gnomes design. The titans use rune-forging technology on their creations which is actually a super-advanced form of programming or circuit forming. Likewise the arcane energy they use in the process and to power all of their creations they do not view as some kind of mystical force like many mortal races do but rather just a form of energy in the universe to use as a power source much in the same way one might use solar power or even fuel. In fact the original reason the titans began traveling out into the universe and forging worlds was solely for the benefit of creating dominated worlds to harvest this arcane energy from.
-Also the original forms of the titans were mermaids. That is why there are pictures of mermaids in Ulduar and why that area has a name like "Hall of Remembrance". Their mermaid forms were what they were on their original home planet, countless millenia after heading out into space they evolved into the form they have now. Or possibly the mermaids were once a planned race for Azeroth that were killed by the Old Gods(either during their first creation or during the war against the Gods) or was a species they had used on other worlds.
Ok now try this one on for size:
Azeorth was the first meeting place between the Titans and the Old Gods, however long after leaving Azeroth the titans began tracking down any other signs of the Old God species in the universe as to remove them. During this time an eternal galactic conflict has existed between the two forces creating the following results across the multi-verse:
-The Flood of the Halo series are actually spawn of the Old Gods. The Forerunners of the same series are creations of the titans who supplied them with their vast technology.
-The titans of Brütal Legend are a renegade group of the Pantheon/titan race that landed on the games world and managed to enslave the Old God minions on it as their servants, before once more departing for the cosmos and putting the events of this games back story in motion.
-The necromorph virus of Dead Space is based on a genetic structure of a spawn of the Old Gods
-The Precursors of the Jak & Daxter series? Titans made em.
-The planet you are on in Alter Echo (remember that game?) is actually a semi-benevolent Old God life form.
-The Reapers from the Mass Effect series are actually rogue AI titan creations that confuse any biological creatures as minions of the Old Gods.
-Lavos from Chrono Trigger? Horrible Old God spawn monstrosity.
I bet none of you knew all those games(and anything involving ancient creator race/Lovecraftian monsters) are actually connected.