The problem with a new Smash Brothers is characters. There will be enough to make a fourth, but not a fifth. Considering that the game has 33 unique Nintendo characters plus the ones from Melee, there is not much left. The only way to make a fifth is to make it a huge crossover or dip into really obscure Nintendo characters.
What? There are hundreds of characters that could be used.
I can list more than 30 series [and representatives] that could still be used, but aren't so far:
Retro:
-Balloon Fighter [Balloon Fight] [Nintendo]
-Bubbles [Clu Clu Land] [Nintendo]
-Dr. Wright [SimCity. It could work, Brawl showed that with his AT and "wand".] [Maxis/Nintendo]
-Donbe [Shin Onigashima, JPN-only] [Nintendo]
-Duck Hunt Dog [Duck Hunt] [Nintendo]
-Lip [Panel de Pon, JPN-only] [Nintendo]
-
Muddy Mole [Mole Mania.
It works better than you'd think.] [Nintendo]
-LoLoLo [Adventures of LoLoLo] [Nintendo/HAL]
-Little Mac [Punch Out!] [Nintendo]
-Mach Rider [Mach Rider] [Nintendo]
-Marina [
Mischief Makers, a N64 game by Treasure, the makers of Sin & Punishment, Gunstar Heroes, and Wario World. I didn't list Gunstar Heroes due to it's current, multi-platform staus, even though it had a GBA-only sequal. She can throw back anything she can get her hands on, even if it's a guided missle or a dragon's fist.]
-Mike Jones [StarTropics] [Nintendo]
-Prince Sabure [For the Frog, the Bell Tolls, JPN-Only] [Intelligent Systems]
-Pulseman [Pulseman] [Game Freak]
-ST Falcon [
Battle Clash] [Nintendo/Intelligent Systems]
-Sukapon [Joy Mecha Fight, JPN-only] [Nintendo]
-Takamaru [Nazo no Murasame, JPN-only] [Nintendo]
Current:
-Captain Rainbow [Captain Rainbow, JPN-only] [skip Ltd.]
-Chibi-Robo [Chibi-Robo] [skip Ltd.]
-Custom Robo Ray Mk. II [Custom Robo]
-Elite Beat Agent [Elite Beat Agents] [iNiS]
-Hero [
Magical Starsign] [Brownie Brown]
-
Icarus/The Hero [Glory of Heracles, a game series that recently has been revived for the DS, but beforehand had 5 JPN-ONLY games for the Famicom and Super Famicom] [Paon]
-
Hunter [
Fossil Fighters, a RPG game similar to Pokemon] [Nintendo/Red Enterainment]
-Isaac OR his son Matthew [Golden Sun] [Camelot]
-Jill [Drill Dozer] [Game Freak]
-Marionation Gear [Chōsōjū Mecha. It got a remix in Brawl.] [Sandlot]
-Mii [Wii] [Nintendo]
-Tom Nook [Animal Crossing] [Nintendo]
-Professor Layton [Professor Layton] [Level-5]
-
Pyoro [
Birds & Beans, a DSiWare spinoff of WarioWare, it already has two games] [Nintendo]
-Saki Amamiya OR his son Isa Jo [Sin and Punishment] [Treasure]
-Starfy [Legend of Starfy] [Tose]
-Unit [Famicom/Advance Wars] [Intelligent Systems]
-Shu [Blue Dragon, two of the three games are on the DS, only the original isn't] [Mistwalker/Brownie Brown]
2011/Future:
-Elza [Last Story, JPN-only (so far)] [Mistwalker]
-Frøy [The Slashing of Reginleiv, JPN-only (so far)] [Sandlot]
-Shulk [Xenoblade] [Monolith, they've re-imagined the two Xenosaga PS2 games for the Nintendo DS [JPN-Only], and have only developed on Nintendo systems since 2006]
-[Unknown Future Protagonist] [Pandora's Tower] [Monolith(?)]
And that's not even including additional characters for any of these [or current] series or 3rd parties. The additions of Capcom, Sega, and Konami could easily triple this list with potential candidates. For example:
Sega
====
-Akira Yuki [Virtua Fighter] Sega]
-Alex Kidd [Alex Kidd] [Overworks]
-Alis Landale [Phantasy Star] [Sega/Overworks/Sonic Team]
-Axel Stone [Streets of Rage] [Sega/Overworks]
-Billy Hatcher [Billy Hatcher] [Sonic Team]
-Bonanza Bros. [Bonanza Bros] [Sega]
-Bug [Bug!] [Sega/Realtime Associates]
-Epsilon-Eagle [Alien Soldier] [Treasure]
-Headdy [Dynamite Headdy] [Treasure]
-Gilius Thunderhead [Golden Axe] [Sega]
-Gunstar Red/Blue [Gunstar Heroes] [Treasure, see above for why it's more assosciated with Sega than their later titles]
-Ichiro Ogami OR Shinjiro Taiga [Sakura Wars] [Sega]
-Joe Musashi [Shinobi] [Sega]
-Kid Chameleon [Kid Chameleon] [Sega]
-NiGHTS [NiGHTS] [Sonci Team]
-Ristar [Ristar] [Sonic Team]
-Ryo Hazuki [Shenmue] [Sega]
-Sketch Turner [Comix Zone] [Sega]
-ToeJam & Earl [Toe Jam & Earl] [Toe Jam & Earl Productions]
-The Ooze/Dr. Caine [The Ooze] [Sega]
-Wonder Boy [Wonder Boy] [Sega]
-Vyse [Skies of Arcadia] [Overworks]
Konami
======
-Bill Rizer [Contra]
-Frogger [Frogger]
-Goemon [Legend of the Mystical Ninja]
-Ken [Survival Kids/Lost in Blue]
-Oolong [Yie Ar Kung Fu]
-Simon Belmont [Castlevania]
-Twinbee [Twinbee]
[A random character] [Silent Hill]
Capcom
======
-Arthur [Ghosts 'n Goblins]
-Amaterasu [Okami]
-Bionic Commando [Bionic Commando]
-Captain COmmando [Captain Commando]
-Dante [Devil May Cry]
-Jill Valentine [Resident Evil]
-Jin Saotome [Cyberbots]
-Hayato Kanzaki [Star Gladiator]
-Mike Haggar [Final Fight]
-Morrigan [Darkstalkers]
-Megaman [Megaman]
-Phoenix Wright [Ace Attorney]
-Ryu [Street Fighter]
-SonSon [SonSon]
-Strider Hiryu [Strider]
-Viewtiful Joe [Viewtiful Joe]
[Unknown] [Faxanadu]
Not to mention that Mario, Pokemon, Sonic, and Megaman can still throw a lot of characters on the table, too.