"about whıch subject shall we talk" is very convoluted. better to say "which subject shall we talk about?" is far more fluent.
Stating "Which" Would suggest you already know what specıfıc choices you have to continue the conversation... Whereas "WHAT" makes the statement open to general debate.
So can you generalise the sentence with "Ne konunda konuşalım?" or does "hangi" in this instance mean both