The planeswalker spark is a mystical quality within someone which, when activated (usually under periods of great stress) gives them fantastic magical powers and the ability to move between planes. The metagene is a genetic quality within someone which, when activated (usually under periods of great stress) gives them fantastic super-powers. The speed force is a mystical dimension which "chooses" people (usually under periods of great stress) and gives them fantastic super-speed powers, which can be used to move between universes. If the Flash ever visited Dominaria, he'd be recognized as a planeswalker -- more specifically, one of the new, post-Time Spiral planeswalkers, who are more like superhumans than immortal gods.