For an extra challenge, limit yourself to one of these roster sizes.
Easy Mode: 32 Characters (+4 Clones)
Normal Mode: 26 Characters (+2 Clones)
Hard Mode: 20 Characters
Heck Mode: 12 Characters (Only 2 series can have more then one rep in this size)
And if you want stages:
Easy: 30 + FD and Battlefield
Normal: 26 or 28 (FD and Battlefield count)
Hard: 20 (again) (FD and Battlefield do NOT count)
Heck: 9 Default with 1-3 Unlocks (FD and Battlefield count again)
I'll try all of these, just for the heck of it.
Heck Mode:
Roster:
Stages:
-Pridemoor Keep (Shovel Knight)
-Scuttle Town (Shantae)
-Mimiga Village (Cave Story)
-Snowdin Town (Undertale)
-The Sovereign (Nefarious)
-Celeste (Celeste)
-City of Tears (Hollow Knight)
-The Salt Factory (Super Meat Boy)
-Dragon Valley (Freedom Planet)
-The Model Village (Untitled Goose Game) (unlockable)
-Floral Fury (Cuphead) (unlockable)
-Fluffy Fields (Slap City) (unlockable)
The Model Village is functionally identical to Final Destination, while Fluffy Fields is functionally identical to Battlefield. The final boss would have its own stage that can't be used outside of that fight.
Hard Mode:
Roster:
Stages:
Aside from the twelve from before...
-Tribalstack Tropics (Yooka-Laylee)
-The Basement (Binding of Isaac)
-Dian Cecht Cathedral (Bloodstained: Ritual of the Night)
-The Ramparts (Dead Cells)
-Mafia Town (A Hat In Time)
-House of Hades (Hades)
-Pueblucho (Guacamelee)
-Streets of New Meridian (Skullgirls)
Normal Mode:
Roster:
(Curly is a clone or semi-clone of Quote; Tostada is a clone of Juan)
Stages:
Aside from the previous twenty...
-Kwolok's Hollow (Ori and the Will of the Wisps)
-Ant Kingdom City (Bug Fables)
-Backyard (Octodad)
-Volcano (Runbow)
-Braid (Braid)
-Windswept Desert (Gris)
Easy Mode:
Roster:
(Fukua is a clone or semi-clone of Filia; Mugman is a clone or semi-clone of Cuphead)
Stages:
The previous twenty-six, plus...
-Field of Hopes and Dreams (Deltarune)
-Club Neon (Katana Zero)
-Showdown (Hotline Miami)
-Tower of Heaven (Rivals of Aether)
Ironically, easy mode was harder for me to complete than heck mode - I wasn't sure whether to expand on the series that I had already added or add brand new series. There were plenty that I could have added, like Otus from Owlboy, one of the knights from Castle Crashers, the Kid from I Wanna Be The Guy, Paco from Brawlout, or the Trapper from Dead By Daylight, but I didn't because I ran out of room. I felt like I could have added more heavyweights and villains. (there's Carrion from Carrion, but I wasn't sure how it would work as a playable character)