They are all similar expressions with subtle differences. 1. 우린 다시 만날 수 없겠지? => I suppose we can't meet again, can we? 2. 우린 다시 만날 수 없지? => We can never meet again, can we? 3. 우린 다시 만날 수 없을지? => Whether we can meet again... 4. 우린 다시 만날 수 없을까? => Do you think we will (not) meet again? 5. 우린 다시 만날 수 없는 거지? => Isn't it true we can never meet again? Between #1 and #2, adding 겠 makes it softer, so #2 is asking it more definitively and conclusively. #3 sounds unnatural in a conversation. It might be a fragment of a thought. #4 might be interpreted in two ways - as a pure question on the possibilities or like #1 and #2, that is, "we can't". #5 is similar to #1, using the "-는 것이지?" form. Other than #3, all sounds natural and reasonable to me.

우린 다시 만날 수 없겠지 was my first pick! i heard the "can" form of it before as well. thanks again :)

Now another expression comes to mind 우린 다시 만날수 없"겠"지? is also used commonly.

thank you! i had heard the 수 있을까 ending before and tried to negate it for the 수 없다 form but seems just a bad guess on my part haha. anyway your advice is very helpful, thanks again!

우린 다시 만날 수 없을까? is used commonly. Have fun learning Korean :)