Spyro the Dragon/WMG

The classic Spyro and Legend of Spyro series take place in the same universe but...
Classic Spyro's world was the one before the TLOS and was destroyed by the Destroyer. TLOS Spyro was the reincarnation of the original Spyro (differences being due to being raised by dragonflies) and succeeds were he fails. Sorry, this has been running through my head this whole time.

Alternatively, the Legend of Spyro really is a prequel to the classic series.
It could also be entirely possible that the Spyro in the classic series is really a decendent of the Lo S Spyro and Cynder.

Building on the above two WMGs, Skyland is the future after classic Spyro.
Legend of Spyro is the past, then the world  and over time changes into classic Spyro. Then Skylanders is the future (It DOES seem to have more advanced technology, what with one of the elements being "Tech.") The three Spyros, and two Sparx and Hunters are reincarnations.

The classic and Legend worlds are parallel to each other
When you think about it. They could be something like yin and yang. The classic world is very bright, cheery, and while there is evil they're normally not as dangerious and threating as most of the sterotypical big bads out there. Meanwhile, the Lo S world is much darker and edgier, with things being realistic in scenery and the villains are much more blood thirsty then the classic villains. It could be said that the main characters of both worlds are the same people, only their destinies being radicially different.

Going off the above WMG, there exsist versions of Cynder, Ignitus, and the Chronicler in the classic universe.
Of course, since the world is majorly different from the Lo S universe they're personalities are probably very different as well.

Malefor IS the original Spyro
Think about it, original Purple Dragon, gleefully taught by his masters, considered a hero, Prowlus even said Malefor and Spyro were very simular as children. Eventually, he went insane from constantly having to save the world (or possibly a villain performing a Break the Cutie on him) and turned evil, and was renamed Malefor in response to his corruption. If true, this would make his Not So Different speech very chilling indeed.
 * This also adds to the creepiness of one of the dragons freed in the first game; he mentions they "always knew", but vanishes before this can be explained.
 * This actually makes a lot of sense...and it also just ruined this troper's childhood.

One of the elements only Purple Dragons can wield in The Legend of Spyro is Fate
It's said the Purple Dragon guides the fate of their era. We don't know how Malefor lead his, but Spyro proves he's pretty much Immune to Fate. The Chronicler said Purple Dragons can use elements others can't, so thus, they can manapulate fate to direct the destiny of their era, subconsciously.

TLOS is actually just a nightmare that Classic Spyro had one night
He dreamed that he became a lousy hero, and that Hunter was huge and muscular. And that Moneybags was nowhere to be seen.

Moneybags is behind everything
He bribed Ripto to lock everyone up, because he knew Spyro would come, and that meant he'd get more money! Plus, he sold Bianca into slavery with the Sorceress because he had her locked up like the animals. Heck! The events of the first game were probably brought upon us by Moneybags.
 * Moneybags probably started the TLOS series too. Maybe even Skylanders.

The reason there are gems lying around everywhere is because they're really low quality.
Moneybags doesn't care, he just sees the shiny gems and wants them now! But the various residents don't care about them because not only do they not need them, the gems simply aren't worth enough- you'd have to go on a huge collecting spree to buy anything, even if there was anything to buy.

The reason Spyro looks so weird in Skylanders: Spyro's Adventure.....
Toxic Waste. Yep. Skylanders Spyro is a mutant.

The Spyro 1 home worlds represent a caste system
Think about it. The way the levels increase in difficulty is similar to how higher castes have stricter rules. The balloon fare is more expensive because it costs more to get to a richer neighborhood. Gnasty Gnorc's world is the last one because he put himself at the top, above all of the dragons.
 * ... except for the fact that the Artisans seem to be the most important and respected dragons generally, and Nestor, the very first dragon you free, is even implied to be some sort of ruler (or at least a senior elder). If anything, the caste system seems to run in reverse order; the highly respected Artisans on top, the Peace Keepers, responsible for law and order, next, the self-important (though not utterly unjustifiably so) Magic Crafters after that, then the slightly country-ish Beast Makers, then the airy-fairy Dream Weavers - and finally the Dragon Junkyard, which truly is the pits of the Dragon Realms. The higher transport fees may simply be because you're going to a rougher neighbourhood each time - more risk for the ballonists (even though they seem to have no problem parking in the previous neighbourhood...).

All of Cynder's elements can be learned by other dragons.
Although the Chronicler calls her abilities "most uncommon", he doesn't call them unique. Additionally, when the Chronicler told Spyro of Malefor's past in The Eternal Night, he mentions that Malefor mastered Wind, one of Cynder's elements. That suggests that Wind, Poison, Shadow, and even Fear actually can be utilized by other dragons (though potentially only by purple dragons).
 * That's quite possible. It's also mentioned that Malefor discovered new Elements, likely before he turned evil. So it's possible.

Skylanders will set up a Crisis Crossover
Think about it. The Big Bad is able to hurl people into alternate worlds. How could they overlook the oppertunity to crossover the three Spyros? Particularally with the outcry from older fans at each new universe. What better way to take advantage of that than featuring all three in one game?

The Dragon Spirits that dragged Malefor to his fate where the spirits of the Purple Dragons between Malefor and Spyro.
Malefor was the first Purple Dragon, Spyro is the present one. We aren't told how old Malefor was, but it seems he's so old that most dragons where unaware that he was a purple dragon to begin with, but Volteer says there were rumors of Purple DRAGONS from old stories, meaning there's been multiple ones. The spirits who dragged Malefor away were purple, but Ignitus' spirit was orange, like him. So they could've been the purple dragons who either opposed Malefor or were killed by him. He clearly didn't succeed with them, as the world wasn't destroyed in the meantime, but he just didn't tell Spyro because it'd have undermined his point if he was the only Purple Dragon to ever become an Omnicidal Maniac.

Skylanders will have more Activion characters than just Spyro
Such as Crash Bandicoot.

Spyro is a Time Lord.
I believe my troper duties are done for today.