A Reply to SCB's Boss Roster
That Petey Piranha comment reminds me of a reply I had in mind.
Before I begin, I'm gonna go out on a limb and assume that Petey P. was the most popular "regular" boss in Sunshine. Gooper Blooper and Bowser Jr. would probably be on that (likely non-existent) list too, but Petey seems more plausible than the squid, and Jr. could get a Ridley reaction (aka "why didn't he get in?"). Since there's a good chance that SSB4 will feature Galaxy themes for Mario, it may be reasonable to have a Mario boss from those games to replace him. A problem with that is the fact that many of those masters use gravity-related mechanics. If the developers are willing to get past that, then could choose from:
(Fiery) Dino Piranha, (Fiery) Gobblegut
Sorbetti
Megaleg, Digga-Leg
Rollodillo
Kingfin
Peewee Piranha
Tarantox
Major Burrows
Glamdozer
Right now, those names are kinda on a...No List. Luckily they have characters who don't use gravity mechanics, like:
Bouldergeist
Bugaboom
Thunder Lakitu
Kamella
King Kaliente
Prince Pikante
Baron Brrrr
Whomp King
Squizzard
Topmaniac
Those turret Towers piloted by a mole. Can't remember the name.
So even though it would be easier to use a boss that didn't have a gravity mechanic, there are some that are possible, like Digga-Leg and others that may not work, like King Kaliente. I think that if they did use a Mario boss from either galaxy game to be a boss in Smash4, the most popular and plausible would make it. In my opinion, Digga-Leg (a large robot with a massive jump from Galaxy 2), Kamella (a giant female Magikoopa from Galaxy 1), and Thunder Lakitu (a massive Lakitu from Galaxy 2) have a good shot. I would choose Bouldergeist personally, but then there'd be a Master/Crazy hand problem.
To wrap this all up, while Petey Piranha is pretty popular and can still work as a master for Smash 4, there's a chance that the Mario Galaxy games will be featured. Because of this chance, using a boss from those games may be more suitable. I can't say who's popular, and not all bosses can be easily placed in Smash. I made some choices, BUT its based off of your idea, and this is just a suggestion.
EDIT: Did I miss any boss names?