
For those who don't want to read the whole thing, here's the tl,dr version.
LoS = AHT/SL
Cynder = Red + Ember
Malefor = Sorcerer
Guardians = fairies + Elders
Gaul = Gnasty Gnorc
Manweersmalls = Blink
Sparx talking
Hunter
LoS environments = AHT environments + steampunk
theory: LoS team thinks AHT = old/original Spyro, evidence? See above.
And now to break it all down. Buckle up, folks, it's gonna be a long one.
Los = AHT/SL
To start off, the producers/writers for the Legend series, Mike Graham and Chris Wilson, did not appear on the Spyro team until the production of Shadow Legacy (a DS title that is the direct sequel to A Hero's Tail; it was originally planned for PS2). That in itself is a bit telling. There are a lot of similarities between LoS and AHT/SL, especially in ANB but also throughout the trilogy.
Cynder = Red + Ember
Before you set fire to my wireless router, hear me out. There are a lot of similarities between these three. Let's start with Ember. Both she and Cynder are the only girls Spyro's age, both are interested in him, both wear collars, they have the same color eyes (green), both were designed specifically to be love interests for Spyro, and they even bear some physical similarities (especially when you look at the body and feet/paws of TEN Cynder and compare them with Ember; tiny feet, cute chunky body). Cynder is essentially what Ember probably would have been if she'd grown up the hard way. Ember is what Cynder would have been if she'd been the darling of her village.
Now for Red. Red was the villain of the first game, obviously, but in SL it is revealed that he was under the control of a dark evil force named the Sorcerer and wasn't ever really evil at all. He is set free and, from my understanding, goes on to help Spyro on his quest to save the world from darkness. Sound like someone we know? You got it. Cynder, too, was presumably never evil of her own accord and was the puppet of a dark evil force, Malefor.
Malefor = Sorcerer
Speaking of Malefor... The Sorcerer was also a dragon who controlled dark forces and sought to have the world swallowed up in darkness. If memory serves, he was even dark purple. He made heavy use of things like Dark Gems, something Malefor was particularly fond of, too. Their motivations are pretty much the same, though Malefor had a case of the crazies from being dead or something, I guess.
Guardians = fairies + Elders
Volter is the yellow Cloudy Domain elder mixed up with Amp, the electricity fairy (he even talks fast). Cyril is the blue elder in Dragon Village (Thomas?) combined with the ice fairy. Terrador is something of his own dragon, but not really. In design, he resembles the dragon elder at the bottom of the well in Winter Tundra (green and brown). His attitude is just the generic old soldier you see in every war movie and bubble breath was lame, so they gave him earth powers. Ignitus is also a bit of his own dragon, but again not really. All of his personality is taken from Obi Wan (even down to the point of him becoming a Jedi Master spirit in the end

Gaul = Gnasty Gnorc
As much as I love Gaul, there's no denying that his design is very reminiscent of Gnasty Gnorc's. Thankfully, Gaul doesn't have big opera diva boobs. But on the point of baddies in LoS, don't they seem kind of familiar, too?
apes/small enemies = small AHT gnorcs etc
larger enemies = AHT larger gnorcs etc
AHT even has pirates and large flying enemies that resembled the Dread Wings!
Manweersmalls = Blink
Love 'em or hate 'em, it's pretty obvious that the Manweersmalls are an homage to the Mary Sue of AHT, Blink the mole. This is also the first hint that Mike and Chris may not have ever played the Insomniac games. There were no notable moles in the original games (the Professor was not a mole in the originals, it was never said what he was and his designer confirmed that he was just a collection of fun shapes to draw).
Sparx talking
I don't recall Sparx speaking human until AHT and he hasn't stopped talking since, it seems. At least he quit with the constant gay overtones in LoS (even went out of his way to prove his straightness by calling Dark Cynder sexy

Hunter
And here's further evidence that the LoS team may not have ever played the original games. Of the older characters who are included in AHT, Hunter was the only one who wasn't overly mauled in character/design and who could feasibly be worked into LoS. Characters like Elora, Sheila, and Bianca, who would have been a good fit for LoS, may have been overlooked simply because they didn't make it into AHT. There's even a lot of similarity between the designs of Hunter in AHT and LoS and it's pretty clear where the artists' who designed him got their inspiration.
LoS environments = AHT environments + steampunk
Admittedly low-quality steampunk, but it's in there. Let's just make a list for this one:
Swamp/Temple = Dragon Village + Crocovile Swamp
Tall Plains/Avalar = Forbidden Jungle
Dante's Freezer = Winter Tundra
Concurrent Skies = Red's Lab
Convexity/Floating Isles = Temple Ruins + Cloudy Domain
etc
theory: LoS team thinks AHT = old/original Spyro, evidence? See above.
I've pretty much highlighted the big reasons why I think the LoS team, or at least the writers, never played the Insomniac games. Any time Mike or Chris talked about the old games, they seemed to be under the impression that they were kiddy games that wouldn't appeal to new generations and which weren't platformers. Ask any non-Spyro fan about the series and they will say something like "Oh yeah, I played those. They were fun. I heard they started to suck. Hey, speaking of Insomniac, do you like Ratchet and Clank?" In interviews, it's almost like Chris and Mike have no idea where these people are coming from when they talk about Spyro being a great set of games. All of the homages to the old games appear to only be of characters who were popular from AHT (namely Hunter, Red, Ember, and Blink), there is a lot of similarity between LoS and AHT/SL in everything from art to story, the writers started working on Spyro when AHT/SL was being produced, the stories are nearly identical (with LoS having had all the fun and brightness sucked out to be replaced with dark and edgy).
I'm running low on steam here, but there you go. Just some thoughts I've had for awhile that make what happened with LoS make a lot more sense to me.
And let me clarify that this isn't LoS bashing. I'm not a fan of LoS anymore, but I was noticing these things long before the other LoS fans (and DotD) killed my love for the franchise. Ultimately, I just find it kind of interesting and also kind of sad that that LoS is really just AHT one or two universes to the side.
Edit: I just found out something else I didn't know about SL. Apparently, that game introduced the concept of purple dragons being special/able to wield multiple elements. Deeper and deeper the rabbit hole goes.