Sweet topic!

Never thought about that concept, might as well share a couple ideas:
Glaciator (Water Brawler) - An alternate version of Eruptor, originally a lava golem which got cought in the rare Eternal Blizzard and lost his fire powers.
He now uses his permafists to fight along the Skylanders, who accepted him after his people banished him from their molten lands.
Temple Rune (Magic Sorcerer) - An alternate version of Prism Break, last member of an ancient tribe of wise and advanced rock elementals, which were said to have found a way to contact the Ancients.
After returning from a 500 years travel across the Skylands he found his village ravaged and the inhabitants deprived from the crystals which granted them their special abilities.
He promptly joined the Skylanders after finding out that it was Kaos who stole the crystals, to experiment with them during the prototype phase of the Evilizer.
Screech (Undead Bazooker) - An alternate version of Torch, she was once an energic and joyful blacksmith who ventured in the Decrepit Dump to find parts for her projects.
Everything changed when she came across an haunted pipe previously used as a component for one of Count Moneybone's machines; the ghost residing inside it forcibly turned the scared blacksmith into a zombie with one of its shrieks but decided to help her by becoming one with its pipe.
The newly renamed Screech joined the Skylanders shortly after with her screeching cannon, to track the Count down and give him the lesson he deserves for not recycling magical parts properly.
Scythe-Fi (Tech Knight) - An alternate version of Grim Creeper, "born" as a normal robotic scarecrow for the Trolls' crops.
Luckily for him his creators were intelligent but not very smart, and accidentally forgot to disable his movement functions.
Once he realized it, he immediately ran away from the fields, ready to start his new life as a free robot and help his stationary brothers as a full-fledged Skylander.
Warmnado (Fire Ninja) - An alternate version of Warnado, this young turtle was found outside the Great Flame Temple.
Raised and trained by the monks to be able to stand on two feet and first, and then fight as a true ninja, he decided to leave the Temple and his peculiar family to join the Skylanders, put his learned abilities to good use and also try to find out where he comes from.
Rocket (Earth Smasher) - An alternate version of Sprocket, this brave but imprudent cave girl had one secret dream: to reach the sky and see the whole Skylands from above.
She tried many strategies, mostly involving stones, but couldn't succeed; until one day where she smashed the ground in anger with her trusty club so heavily that the shockwave sky-rocketed her over the clouds.
After Rocket understood the marvels of the sky, and the problems of landing, she decided to share her discovery with the world starting with the Skylanders, who saw her potential and asked her to join them... At a safe distance!
Wind-Spell (Air Sentinel) - An alternate version of Wham-Shell, this friend of Hermit is, just like him, afraid of clouds: which is quite a problem if you are a celestial crab.
Wherever he went, those menacing clouds always followed him threatening to unleash their fury on him.
One day, fed up with his fear, he decided to seek a way to get rid of clouds once and for all; after days of research in the Spell Punk Library he finally found a powerful scroll which could allow his staff to rotate at high speed, to generate a gust of wind capable of wiping out even the biggest and darkest cloud.
Needless to say, Hermit insisted that Master Eon made such a strong hero join the Skylanders: even the most patient Portal Master has to make deals, sometimes...
Vinegrowth (Life Swashbuckler) - An alternate version of Nightfall, this solitary forest druid used to like gathering flowers and herbs in the Misty Meadow.
One day she ventured in a place where she were never been before, a strange circle of huge stones with a small shrine in the center: curious, she came closer to it and discovered two oddly sharp leaves.
As soon as she touched one of them, the stones begun shaking and revealed themselves to be the legs and arms of a gigantic golem which was forming right before her eyes.
Surprised, but not scared, Vinegrowth immediately sliced the vines which kept the golem together, defeating it before it could be ready to attack.
Once the monster was turned back to rocks, Master Eon appeared next to the druid: he told her he witnessed the whole scene and asked the druid if she wished to fight again, even suggesting a special nickname to celebrate her success.
Voidart (Dark Bowslinger) - An alternate version of Astroblast, this gloomy and mysterious hooded archer was once the terror of the night travellers.
He was capable of waiting for hours for a victim and the shoot one single arrow to scare the poor merchants away and recover their treasures.
This lasted until Voidart found out there was a much more frigntening thing than him, in the Skylands: The Darkness.
Shocked for this sudden revelation, he reached the Skylanders Academy and promptly asked Master Eon to help him defeating it.
There is space for only one terror, in the Skylands, and he surely didn't want to be the one left out!
Bright Ray (Light Quickshot) - An alternate version of Food Fight, this living light sparkle begun his exsistence shortly after Cloudcracker Prison blew up and the Light Element returned to the Skylands.
The explosion sent him in the middle of a battle between a group of Trap Masters and some recently-escaped villains.
Not knowing which side he was on, Bright Ray begun attacking both... And ended up winning!
After a couple explanations from Master Eon, he quickly understood how things worked and happily joined the Skylanders to repeat such a fun experience.
Who knows what would have happened if Kaos talked to him, first...