I want to see if なにかな or なんだろう, would be used in a similar way. I asked about what would be a good chat topic and the response was なにかな. I have also seen なんだろう and jisho gives similar definitions. Don't both terms indicate a type of wondering? I am thinking that なんだろう may be more of a surprise.
なにかな is a word to tell someone.
なんだろう is a word to tell yourself.
When you are talked to by someone, you should say なにかな to him or her and you should say なんだろう to yourself.
なにかな and なんだろう means “What do you want to say?” “What is that?” “What?” “ I wonder what.”
In some Japanese TV programs, a comedian has to answer what is in a box (It may be a snake or other animals.)
He says なんだろう. He is just saying “I wonder what’s in the box.” not particularly paying attention to people. He also says なにかな. He is telling people there “I’m wondering what’s in the box.”