I thought about this for awhile, and I really think the overall idea of Smash Bros has kinda been lost among a lot of people. The idea of Smash Bros is an All-Star fighting game, meaning their best of the best characters duking it out.
Ok, sure, we're getting close to the bottom of the barrel of characters to choose from, but think about some of the characters that people suggest (i'm not going to name any particular ones so I don't start a flame war). Are some of them really "all-star" characters? Would it make sense for them to be on a roster with the likes of Mario, Link, Kirby, Pikachu, or even Fox?
Let's take a few examples:
Ridley- Clearly an All-star. Well known Metroid villain, fairly popular to the Smash community in at least the US. There is no other character from Metroid more deserving of a spot.
K.Rool- Villain of the Donkey Kong series, well known at that. Arguably competing with Dixie Kong, but he's definitely more popular.
Palutena- Well liked character in newest installment to Kid Icarus. Very important to the plot, a main character to the series.
Mewtwo- Icon of the entire Pokemon franchise. Already in Melee, and is still a very popular choice, arguable most popular for Smash 4.
Shulk- Kind of different compared to the others, but a popular character nonetheless. While he only has one game in his series, and may or may not appear again, Shulk is well liked and could prove as an interesting character. Could Shulk be considered a Nintendo All-Star?
All I mean is, that fans will get their hopes up if they want an incredibly obscure character or idea. Again, i'm not going to name certain characters, but there are plenty of characters that most likely will not make it just based off of the competition ahead of them.
End of wall o' text.