When I saw the README document for Crusade, I saw that you can add your own versus themes. I decided to try it out. Just know that Klonoa, Krystal, Mach Rider, Mr. Game & Watch, R.O.B., Shadow, Shantae & ZSS don't & probably won't have their own versus themes. As for now, on with the list! (REALLY long) (All music belongs to their respective owners)
Ashley: Ashley's Song - WarioWare: Touched!
Bomberman: Bomberman Blast - Main Theme
Bowser: Bowser's Theme - Mario & Luigi: Dream Team
Captain Falcon: F-ZERO Medley - Super Smash Bros. Ultimate
Chun-Li: Chun-Li - Street Fighter 2 Turbo
Crash Bandicoot: Crash Bandicoot N. Sane Trilogy - N. Sanity Beach
Donkey Kong: Kongo Jungle - Super Smash Bros. Melee
Dr. Mario: Dr. Mario Chill - Tetris & Dr. Mario
Duck Hunt Dog: Duck Hunt Medley - Super Smash Bros. Ultimate
Evil Ryu: Theme of Evil Ryu - Ultra Street Fighter IV*
Falco: Space Battleground - Super Smash Bros. Brawl
Fox: Corneria - Star Fox Zero
Ganondorf - Ganon Battle - The Legend of Zelda: Twilight Princess
Geno: Geno Awakens - Super Mario RPG
Goku: Goku's Theme - Dragon Ball FighterZ
Gooey: Grass Land Map - Kirby's Dream Land 3
Ice Climbers: Ice Climber - Super Smash Bros. Brawl
Jigglypuff: Battle! Wild Pokemon - Pokemon Omega Ruby & Alpha Sapphire
Kirby: Staff Credits - Kirby's Dream Land - Super Smash Bros. Ultimate
Knuckles: Unknown from M.E. - Sonic Adventure 2
Link: Hidden Mountain & Forest - Super Smash Bros. Brawl
Little Mac: Jogging/Countdown - Super Smash Bros. for Wii U
Lucario: Battle! Dialga/Palkia - Pokemon Diamond & Pearl
Lucas: Unfounded Revenge/Smashing Song of Praise - Super Smash Bros. Brawl
Luigi: Luigi's Mansion Theme - Super Smash Bros. Brawl
Mario: Mario Circuit - Mario Kart 8*
Marth: Attack - Super Smash Bros. Brawl
Mega Man: Mega Man 2 Medley - Super Smash Bros. for Wii U
Meta Knight: Duel with Meta Knight - Kirby Super Star
Mewtwo: Victory Road - Super Smash Bros. Brawl
Nega Shantae: Boss Battle - Shantae: Risky's Revenge
Ness: Onett - Super Smash Bros. Melee
Olimar: Main Theme - Pikmin - Super Smash Bros. Ultimate
PAC-MAN: PAC-MAN - Super Smash Bros. for Nintendo 3DS
Petey Piranha: Endlessly Hungry, Petey Piranha Battle - Paper Mario: Sticker Star
Phoenix Wright: Pursuit - Cornered - Phoenix Wright: Ace Attorney
Pichu: Poke Cup Final Battle - Pokemon Stadium 2
Pikachu: Pokemon Battle - Super Smash Bros. Melee
Porky Minch: Porky's Theme - Super Smash Bros. Brawl
Rayman: Rayman Origins - Gourmand Land - Glacier Cocktail
Ridley: Vs. Ridley - Metroid: Other M
Ristar: Act Clear - Go Ahead - Ristar
Ryu: Theme of Ryu - Street Fighter V
Saki Amamiya: Within Earshot - Sin & Punishment
Samus: Theme of Samus - Metroid Samus Returns
Snivy: Route 10 - Pokemon Black & White
Solid Snake: Encounter - Super Smash Bros. Brawl
Sonic: Sonic Boom - Sonic the Hedgehog CD
Tails: Believe in Myself - Sonic Adventure 2
Tingle: Termina Field - The Legend of Zelda: Majora's Mask
Toad: Toad's Theme - Mario Strikers Charged
Toon Link: Title Screen - The Legend of Zelda: Tri Force Heroes
Waluigi: Waluigi Pinball & Wario Stadium - Mario Kart DS
Wario: Title - WarioWare: Smooth Moves
*1: Evil Ryu's new versus theme is the same as the original. I have no idea on why I put it there. (
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*2: Mario's versus theme is subject to change.
Now share your own versus themes for classic mode! (Must have character name for each theme you put.)