Hannibal Roy Bean makes it clear that Jack was to be the best villain as soon as he overcame his fears. Xiolin Showdown is unique in that they like turn over typical adolescent targeted demographic cartoon cliches in favor of good storytelling. For example making Raminudo the leader in the end of the series instead of Omi. In Jack's case, he actually was and is a baddass. Just young, inexperienced and with certain vices (laziness, pride, cowardice) that always made him a step behind the monks or other villains. In time, and with maturity, I don't doubt that even with Omi as part of the timeline Jack still is destined to become a formidable mad scientist/esoteric magic wielding threat.