My vague hope is that the burning Smash ball is actually part of a story mode, perhaps being a portal that brings all the fighters together. They then join forces to find the cause of it all.
You could have a trailer of each Smash Universe finding it and touching it, only to be transported to the same realm. Mario and the gang fighting Bowser only to be interrupted by the burning orb, Fire Emblem heroes from across the centuries stumbling upon it, Kirby and friends venturing towards it, etc. Anyways, just my fever dream