They really gotta add more characters for under represented /obscure Nintendo franchises, so they can maybe get ported over seas like Fire Emblem or revived like Kid Icarus. Golden Sun and other lesser known Nintendo series deserve another shot!
https://imgur.com/a/jLkRgOb
Nintendo's vast amount of series is what makes them a cool company! No offense to the people who want shy guy, chain chomp, goomba, mario is missing luigi, 8 bit mario, koopa, koopa troopa, thwomp, etc. :b
There's a lot of great franchises and franchise entries from the GBA/GameCube era that didn't get a character, stage, assist trophies, etc.
We got essentially every major and notable Nintendo-owned character from the NES/Famicom era, SNES/Super Famicom/Game Boy era and the N64/Game Boy Color era already playable (except Skull Kid, Paper Mario, Dixie Kong and maybe Cranky Kong; like that is actually "the list" of characters of that era that ever had any mass appeal).
The GameCube/GBA era is the next frontier of "retro Nintendo-owned characters." We've burned past the bottom of the barrel in regards to those other eras (again, the aforementioned exceptions aside).
- In Melee we got Roy from that era.
- In Brawl we got Ike, Lucas, Olimar and Toon Link (and kind of the Pokémon Trainer's new-ish design too + Zero Suit Samus' and Wario's new look) from that era.
- In Smash 4 we got Bowser Jr (and sort of Villager too; technically debuted on the N64 but that was Japan-only) from that era.
- In Ultimate we got Dark Samus so far.
Isaac I think is "the big one" in regards to that era that is missing. Other notable characters include a Gen 3 Pokémon (Blaziken probably, Sceptile, Gardevoir are other possibilities as well), something Advance Wars (not having a real super central character hurts; Andy is the closest thing to that), and Chibi-Robo.
There's about as many characters from the Wii/DS era left as well. The current gen (RIP Wii U/3DS/Switch era) is the gen with the most quality-ish Nintendo-owned newcomer candidates actually.
Also fun fact: In regards to 3rd party playable characters, 1 debuted in Gen 2 (Pac-Man), 4 debuted in Gen 3 (Solid Snake, Mega Man, Ryu, Simon Belmont), 2 debuted in Gen 4 (Sonic, Richter Belmont), 1 debuted in Gen 5 (Cloud Strife), 1 debuted in Gen 7 (Bayonetta).
Zero debuted in Gen 6, aka the GameCube/Game Boy Advance era (which is shared with the original Xbox/PS2/Dreamcast). Sora from Kingdom Hearts, Dante from Devil May Cry and Phoenix Wright are probably the most notable characters that have a chance to debut in that era (also Master Chief from Halo if Sakurai and co are insane and try to end crossovers from ever topping this game ever lol).
It would actually be hilarious to see Isaac and one of those 3rd party characters playable and fighting, especially Master Chief lol.