List of Bosses that would work very well in Smash Bros' Battle Boss mode
Porky - Same as Brawl
Petey - Petey need quite a lot of adjusts to become his own boss, rather than holding two cages
Rayquaza - Same as Brawl
Ridley - Clone of Ridley being a boss battle (work just similar as Brawl)
King K Rool or Sir Frederick - Either works very well
Dark Matters or Marx
Black Knight - Superarmored Ike clone
Medusa or Hades
DJ Octavio - function just like Splatoon 1 Boss Battle where you have to avoid getting hit by the punches, missiles, etc. And you have to hit those hands to send it back to the boss.
Andross - but function differently than DJ Octavio but you have to attack the hands to able to get his head into a daze in order to damage it
Black Metal - same as stage hazard in Gaur Plains, and they could adjust something to suit as a boss battle
Ganon - they could borrow the assets from Ganondorf's final smash, and come up with interesting movesets for a boss-liek battle.
Giga Bowser - Bring Smash 4's Giga Bowser back as a boss battle
Sephiroth - swift swordifghter with a lot of quick dash with slashes, etc
M. Bison or Akuma - just like how they fight in the game (as secret boss or boss)
Dr. Wily
Dr. Robotnik
Lucien from Bayonetta
Someone from Metal Gear Solid
Ghosts from Pac Man would be difficult to put together where you have to learn how to dodge those ghosts until power pellet popped up so you can damage the ghosts