I don’t know if we’d want these, especially for the initial bingo, but just thoughts I am avoiding work, yes. Maybe they can inspire more prompt suggestions.
Challenge modifiers 
- Take on the bingo challenge with only one type of book (manga, LN, novel, etc.)
- Take on the bingo challenge reading only books from your backlog
- Take on the bingo challenge without reading any books from your backlog (RIP your backlog
)
- If any of the books you read has a different media adaption, also consume that media adaption
- Read only books that no one else has (at the time) used for their challenge
- No books you fill out the challenge with can be the same level
- All books you read are either physical or digital; no mixing
- Each book title/author read must start with a different syllable (e.g.: can’t read two books whose titles start with か)
- (from @Mara) Bingo entries must abide by shiritori rules
- Extra spicy: your final book title must end with ん
- Fill your bingo card with books from just one genre
- Don’t use your free space
- (from @SophieShiori) Finish your bingo with as few books as possible (one book counting for multiple spaces)
- (from @mita) Extra spicy: read a single book that qualifies for an entire bingo line