In the original post, there's a list of potential themes that everyone may do. (For example, there's a lot of Punch Out!! contenders milling about here, which was planned here.) If you have a theme to suggest or a character to claim, then you just have to say what the theme is and who you want to claim for said theme. There's a list of who's been claimed for what in the second post.
Whatever the next theme is, I won't be participating. I already know what my May theme is going to be, provided I'm still in a Brawl Stars phase then. (No, I won't tell you who I'm going to be.)
Couldn't find any good fanart, so looks like I'll be drawing an avatar myself this month! I have a few other higher priority things to work on right now though so it'll probably be a few days.