Elle Fanning looks like a good choice to play a young Effie Trinket.
I haven't read any of the books, but I've seen all the films to date. My view of Effie was one of someone who was a bit of a failure. I don't think it's her fault, although it could be, more just always on the outside of the in-group. She worked her way up but the best she could do was District 12. For her it's everything, but to everyone else it's a joke.
Which is to say I think she could have an interesting story, but not one that can be told the same time as Haymitch. I get more of a fan service vibe vs this is the right time to tell her story vibe.
I'm not looking for spoilers, but curious to know what others think.