I'm going to try some Mass Effect spirit ideas.
Commander Shepard (male and female) would be the fighter spirits as two separate entries. Not sure who would be the default. (the game defaults to male, but my personal preference would be female, largely due to Jennifer Hale's voice acting)
I'm not going to mention the music, since it's been a while since I played Mass Effect, so I can't recall the music, and most of it would be atmospheric/ambient and (from what I recall) seems like it wouldn't be fitting for Smash.
Bear with me - it's a while since I've gone in-depth when it comes to playing spirit battles in Smash Ultimate, so I'm not sure which effects generally go with which types of spirits. It's also been a while since I've played Mass Effect, so some spirits might be stretches.
Garrus Vakarian
Spirit Type:
Primary - Shield
Opponent(s):
Stage: Bridge of Eldin
Conditions:
-enemy starts with Staff
Effect: Critical Hit Up
Tali'Zorah vas Normandy
Spirit Type:
Primary - Neutral
Opponent(s):
Stage: Shadow Moses Island
Conditions:
-defeat main enemy to win
Effect: Energy Shot Resist Up
Liara T'Soni
Spirit Type:
Primary - Shield
Opponent(s):
Stage: Great Cave Offensive
Conditions:
-Daybreak item spawns
Effect: Shield Durability Up
Urdnot Wrex
Spirit Type:
Primary - Attack
Opponent(s): red
Stage: Coliseum (Battlefield)
Conditions:
-opponent likes to use Neutral Special
-the floor is poison
Effect: Neutral Special up
Ashley Williams
Spirit Type:
Primary - Neutral
Opponent(s):
Stage: Great Bay
Conditions:
-enemy starts with Ray Gun
Effect: Ray Gun equipped
Kaidan Alenko
Spirit Type:
Support
Opponent(s):
Stage: Temple
Conditions:
-enemy throws powered up
Effect: PSI Attack Up
Jeff "Joker" Moreau
Spirit Type:
Support
Opponent(s):
and metal
Stage: Lylat Cruise
Conditions:
-defeat the main opponent to win
-main opponent is easy to launch
Effect: Irreversible Controls
Thane Krios
Spirit Type:
Primary - Grab
Opponent(s): green
Stage: Frigate Orpheon
Conditions:
-enemy will occasionally turn invisible
Effect: Throws powered up
Legion
Spirit Type:
Support
Opponent(s):
Stage: Palutena's Temple
Conditions:
-enemy starts with a Staff
Effect: Staff equipped
EDI
Spirit Type:
Support
Opponents: metal
Stage: Corneria
Conditions:
-the enemy starts the battle with a Super Scope
Effect: Metal Box equipped
Javik
Spirit Type:
Primary - Attack
Opponent(s):
Stage: Brinstar
Conditions:
-enemy starts with a Rage Blaster
-both players will suddenly take damage
Effect: Rage Blaster equipped
Saren
Spirit Type:
Primary - Shield
Opponent(s):
(once beaten,
spawns)
Stage: Kalos Pokemon League
Conditions:
-hostile Assist Trophies (Andross) will appear
-low gravity
Spirit can be upgraded to...
Saren Husk
Spirit Type:
Primary - Shield
Effect: Jump Up
The Illusive Man
Spirit Type:
- Support
Opponent(s):
Stage: Final Destination
Conditions:
-the enemy tends to avoid conflict
-hostile Assist Trophies (Gray Fox) will appear
-defeat the main enemy (Robin) to win
Effect: Item Gravitation
Harbinger
Spirit Type:
Primary - Neutral
Opponent(s):
plus
x 3
x 3
x 3 (starts off with giant Ridley; enemies keep spawning in, with a maximum of five characters on-stage at once)
Stage: Midgar
Conditions:
-main enemy is giant
-main enemy has super armour
-defeat the main enemy to win
Effect: Super Armour