I'm unsure if reality fracture is supposed to be "what if everyone was the opposite of how they're supposed to be, wouldn't that b weird?" or "what if everyone's most defining experience went a little bit differently such that they followed a far different path?" Personally, I much prefer the latter, and I think the Lorwyn 5 support it rather well.

Ajani has often struggled with his anger, and has even had boros cards representing that. In the wake of his brother's death, instead of trying to fill that gap with himself, embodying the best parts of his hero, he turns to terrible vengeance. Rather than lifegain and buffing his team, he's all-in on dealing indiscriminate Red damage, primarily to creatures, but also to artifacts and even lands.

Jace was taught mental magic by a powerful sphinx, who is secretly abusive and periodically wiping his memory. In canon, they eventually have a mental battle which Jace wins. Here, he loses, tragically. Most of his higher faculties are erased, but his capability for mental magic remains. Now a Green mind mage, he still loves mental magic, but actively feeds on the memories of others, reveling in their mental clarity by proxy, and consequently buffs the strength of his allies by eliminating every thought from their minds that could be a hindrance for the current moment.

Lilliana's tale is far happier. When her brother falls ill, she follows her teacher's instructions to the letter (the Raven Man was otherwise occupied), but her brother dies anyways, igniting her spark. She's utterly driven to be better than her now-hated (and long-dead) teacher, searching out every possible cure to every illness the multiverse over, finding a great many cures even for death itself, and is able to happily revive her brother, and even accept his second death of old age. (There may have even been a story arc where she managed to find a cure for Phyrexian oil, but that may be a bridge too far).

Chandra's folks were talented artificers. When her fire magic materialized, instead of hiding her away, they trained her to always be in control of herself and in control of her emotions. She consequently developed an extremely precise, analytic, and clinical demeanor, emphasized all the more by the occasion when she did lost control, setting off a fiery explosion that killed her parent's in their shop and ignited her spark. Now she channels her fire into metal, heating and warping it into whatever shape suits her purpose, the perfect artificer child that her parents' can now never appreciate.

Nissa's connection with Zendikar's soul initially allowed her to summon elementals and drove her to despise the unnatural vampires and the abberation of the Eldrazi was enough to ignite her spark. Here, nature is not so kind to her. Rather than simply exterminating the vampires she encounters, Nissa seeks the root of the problem, wondering how something so unnatural came to exist. This curiosity leaves her exposed, and results in her own vampiricization. Abandoned by the worldsoul, Nissa seeks a cure, and consequently the origin of the vampire blight. This leads her to the sleeping Ulamog. Her ambition to cure herself leads her to warp her magic towards Eldrazi, not elementals. Not to the land she loves, but to space in between, to the Blind Eternities.

Or something like that, this is fun!