Amazing job with the sprite work. Also, I did see the tweet you posted on characters suggestion so here's are my pick, power-ups, and character cameos (these are the characters that you didn't add to your list).

Characters: Dirge from Xombie Dread on Arrival, a Sword and Sandals character (could be the main character or any character but I would pick the main character from the third Sword and Sandals because that's the game that has the best design IMO), Hobo from Hobo, Auditor from Madness Combat (the reason why I picked Auditor because he is the main villain in the series with a unique moveset and I did see some people requesting him), Darnell from Pico School, Matt from Epic Battle Fantasy, Armed with Wings character (I don't know too much about the game but I like the character design), and I know I've requested this character before but Ami from Gwain Saga as a single character but if she has the final attack (a knock off idea for the final smash) where she would summon her allies/friends "Deva, Agni, Marlow, Lanney, and Savien to do an all out attack at the opponent and Gwain and Luna from Gwain Saga as doubles character similar to how Pyra and Mythra gimmick plays. I know not adding Geo might sound like a bad idea but I have to be unbiased as possible when making a list and also this isn't the first time where the main character wasn't added in a fighting game. Street Fighter has done it with Rival Schools and most people wanted Akira Kazama to be in Street Fighter V instead of Batsu because she was the fan favorite and when the team announced that there were going to add a Rival Schools character they went with Akira Kazama, the fan-favorite character from Rival Schools. Heck, even Smash Bros Ultimate did it with Min Min and Pyra/Mythra instead of the main character because they were fan favorites from the series.

Power-Ups: Level 50 icon being the mysterious question mark box that will give the opponent a mysterious item. Level 49 icon would eliminate the opponent when at very low health. Level 43 icon would give the opponent a bad item and do some damage at the opponent isn't so careful. Level 59 would increase your damage. Ami cat ears (it's a wearable item that Ami wears to heal herself in the series) would heal slowly to the opponent and increase your speed. And the lower level icon would be small healing items like the food icon for example. Now I know these characters are already on your list but I would add Turtle Toss as an item similar to the soccer ball in Smash Bros Ultimate where if you hit it, he would hit the opponent doing an insane amount of damage, and Interactive buddy as an item and a sandbag for the training mode.

Character cameos and other stuff: Cassandra should join the villain group in your roster because she was the OG villain from Pico School. I would also love to see her summon characters from Pico School such as Hanzou, Cyclops, Alucard, and other small characters from Pico School and Pico School 2. Hank should have another stage where you could see Sanford and Deimos as character background and taunts for Hank.

I can go on and on with this idea but I already spent like an hour making this list lol so I'll replied again for assist character's, costumes, and bosses ideas soon.