MSPaint drawings of bullet patterns, bomb concepts and ideas (anything is possible at this point), any special abilities or traits you may want the player to have.
For example, I had an idea for one of the Zengar player characters: If an enemy gets too close, Zenger will use his sword to deal extra melee damage to the enemy (like shotgun bonus damage or something). Of course there would be a neat animation or something to go with it. Just an idea.
Another example:
Like Kanako's sketch. I was kinda hoping for something more batshit though, instead of boring old basic forward shots
Anything can be put on the table.