Oooh, that's a fun idea! Instead of giving you a list, I wanna showcase one boss in particular that I think would be a ton of fun to fight against in Smash. The Computer Virus from Kirby!
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You start off fighting the Magician, while the text bar narrates your moves as well as the opponents moves. The text bar also states how much damage you've given to the opponent, and vice versa; ex: The Magician takes 72 damage!
Once the Magician is defeated, you move onto the Evil Knight. And then finally, a Great Dragon.
Not only is the Computer Virus one of my favorite bosses from Kirby, I just think it'd be really cool to have an actual RPG-esque battle in Smash. For me at least, it'd just be a lot of fun as like a "halfway through bossrush mode boss" kinda deal. Nothing too serious, not too hard, but it can be difficult if you arent careful.