Here's my wishlist for characters to be in Mugen:
Non-JUS
Yuji Itadori
Ken Kaneki
Gon Freecss & Killua Zoldck
Tanjiro (as a main) & Nezuko (as an Assist) Kamado
Denji (although there might be a full-sized char out there)
Melodias
Most of My Hero Academia characters (particularly Izuku, Bakugo, Shoto, and the Pro Heroes)
Rimuru Tempest
Eren Yaeger (there's a full-sized version of him based on his Titan form by chuchoryu but it isn't good)
Shinra Kusakabe
Okumura Rin
Asta
Nel
...And more to come!
Cloud Strife (based on the FFVII remake or Smash Bros.)
Naruto (based on the later chapters/seasons)
Ryu Hayabusa (based on the reboot; not from chuchoryu)
K.O (from OK K.O)
Ichiban Kasuga (from Like A Dragon/Yakuza)
Yor Forger & Fiona Frost (from Spy x Family)
A proper Kazuya Mishima (mostly based around 7 or 8; from Tekken)
Chris Redfield (based on Resident Evil Village)
Elphelt Valentine (from Guilty Gear)
Ms. Fortune (from Skullgirls)
Gumball Watterson (based on later seasons)
Plankton (might be controlling a robot like Tron Bonne)
Erik Killmonger (based on his MCU appearance)
Esdeath (from Akame ga Kill!)
Revy (from Black Lagoon)
...And more to come!
As bosses (more to come):
Bill Cipher
Femto/Griffith
Nyarlathotep
Aku (from Samurai Jack; there is one character somewhere but I'm not going to state it)
Flowey/Asriel Dreemurr