It means that we will get in touch in due time, or soon enough. As I see it, it is a way of confirming with your interlocutor a consequent exchange that will provide some missing piece of information concerning the matter in hand.
Some possible situations are: you can't remember something off the top of your head, you still don't have the info but expect to have it at some later time, further communication is expected to complete the arrangements, etc. It could simply mean: "I'll let you know about the details".
I would say it has a reassuring element of meaning to it most of the time, but, realistically, it depends on the tone of voice.
alexl57's answer is correct. I still need to give a further emphasis on tone of your voice and the occasion, which can mark a completely different meaning, "hostile", as you said. "Čućemo se već!" can easily mean the temporary end of an unpleasant exchange, with a promise that the opponent can expect further escalation from your side.
Do you mean something along the lines of "I will hear from you for sure" or "You will hear from me, no doubt about it" .
It would be interesting to see how is this translated into English normally! I think it's possibly more of a Croatian way of saying.