If this ever gets a follow up, some ideas:

• Bowser gets replaced with Giga Bowser
• K Rool stays the same
• Ganon takes on his appearence from Ocarina of Time, with a bit of Calamity Ganon surrounding him
• Dedede looks to be fighting the Dark Matter within, with Zero looming behind him
• Medusa takes on her new form from Kid Icarus Uprising, with Hades trying to butt in
• Porky switches to his spider mech from Mother 3, with Giygas looming behind him
• King Boo laughs at Luigi's fear
• Polterpup accompanies Luigi, growling at the new threats