I like this idea much more. I'm not going to say Skull Kid has no chance of being a playable character, but some people suddenly assume we'll get Skull Kid because Young Link is playable as well, despite Ultimate just bringing back every character that was playable in the past. Skull Kid not being an assist trophy can point into the direction that he'll be playable, because that's what a lot of people guessed with Isabelle missing as an assist trophy. I don't know which bosses we'll see aside from what has already been shown, so from the original 12, here are bosses I'm hoping are in Ultimate:
Super Mario Bros.
Wart
I tried looking up different bosses Mario (and Luigi) have fought throughout their games, and Bowser seems to be the main villain in all of them, which of course makes the most sense. However, I could see Mario and Luigi finding themselves in a world, Subcon, and them having to go through the map and eventually encounter Wart. Once they defeat him, the two are sent back to their own world.
Donkey Kong Country
Tiki Tong
He's probably not the most memorable or most interesting boss to the DK series, but seeing as how King K. Rool is now a playable character, I wasn't sure who to add here. I suppose he'd play in a similar fashion as he does in Donkey Kong Country Returns
The Legend of Zelda
Agahnim/
Majora's Mask
Agahnim would probably be a nice choice here since he's just the alter-ego of Ganon himself, and we know Link and a younger incarnation of himself has dealt with Ganondorf/Ganon, this would sort of make sense, since the Hero of Time himself never fought Majora as an adult. Still, because of the possibility of Majora being a boss, I suppose it would make sense to stick with him if he's not playable, since Ganondorf is already a playable boss character.
Metroid
Mother Brain
I'm talking about her second form, of course. She'd be a difficult foe to face off against, especially if she started using her Laser Brain Attack that could have killed Samus. Unless they went about making Melissa Bergman a boss and she ended up somehow transforming into the second form of Mother Brain. I don't think that take would be popular, however.
F-Zero
Black Shadow
This would probably be considered a silly concept, but perhaps Black Shadow has schemed to blow up something important, that as of right now I can't think of. Maybe some characters you're going to have to save. Instead of racing him on the tracks like you normally would, it's a race against time. Time would still be going on once you reached Black Shadow, and of course the only way to stop the explosion would be to defeat Black Shadow.
EarthBound
Pokey Minch
This was a no-brainer. Lucas never had to deal with Giygas, and another thing the two PSI boys have in common, aside from PSI, stripe shirts, and blue shorts is that Pokey is a character I'm sure have come to despise. He'd be in his heavily armored spider mech, just like he was in Brawl, most likely acting in the same manner. Unless of course he becomes a playable character himself, then I guess add Giygas as the boss. I'd like to see that in 3D.
Yoshi's Island
Petey Piranha/
Navel Piranha
Either we get what we had in the Brawl version, which was cool, or make him do something different. He'd be able to fly around and then ground pound. Instead of shooting goop from his mouth like in Super Mario Sunshine, he could shoot fireballs. After you take out his first form, Kamek flies in and uses his wand to give Petey Piranha a new form, Navel Piranha. In this form, he'd have tentacles that he could whip around at you, grab you and toss you around, and even a head butt attack like in Yoshi's Island.
Kirby's Dream Land
Dark Matter
Kirby and the gang approach King Dedede, and they notice he seems a little off. King Dedede runs straight at Kirby, swinging his hammer around, and Kirby ends up counter the attack with a simple kick. King Dedede ends up falling down, and Kirby and the gang think, "Well that was easy," only to see some black powdery stuff coming out of the penguin. Dark Matter appears with sword in hand and covered by a cape. That's when the boss battle begins. After you deal with his first form, he turns gets rid of the sword and cape to show his true self, and attacks you with the orange good, and fires black lightning bolts at you.
Star Fox
I'd say Andross, but I'm not really familiar with this series, so perhaps someone else could mention another meaningful boss.
Pokémon
I stopped playing this series a long time ago, so I'm not sure who the most formidable Pokémon is in the current generation.
