A fixed dropdown list of flag reasons would be a very good change, I think, because it would somewhat counteract strategic flagging of stories as a downvote. I think you'd want to keep the list pretty tight, because it seems like a huge source of meta drama.

The problem with fixed lists is that what is in the spirit of HN evolves.

Single page info-graphics and Awesome-Lists come to mind.

Hell, one or more of PG’s books has one or more baysian generated texts presented as poems.