For example I find myself saying things like:
What did you win, bro?
I could leave out the "bro" and the sentence would be complete but I unconsciously add emphasis with the subject again because it's the way I speak.
In a conversation, would it be okay to do the same in Japanese for emphasis as well? Something like:
何を当てるの[兄貴]{あにき}?
I'm unsure since in Japanese you can omit the subject altogether. Or maybe it just doesn't sound or translate the same, thoughts?
As virmaior said in the comments, "bro" in this sentence is a vocative - it's used to address "bro". It's not the subject of the sentence. The subject of the English sentence is "you".
Vocatives' natural position is at the end or beginning of a sentence. In writing, you usually put a comma to separate it. When speaking, you make a short pause.
Thus
What did you win, bro?
to me looks better written as
何が当たったの、兄貴?
(Note, I changed the structure of the sentence based on 非回答者's comment below.)
As for omitting or adding a vocative, yes, you can use it or not, not very dissimilar to English:
何が当たったの、兄貴? What did you win, bro?
兄貴、何が当たったの? Bro, what did you win?
何が当たったの? What did you win?
