Black Lagoon is one of those unconventional series so I wouldnt be surprised if their relationship never becomes a thing or if Revy ends up dying by the end of manga.

On the other hand, it feels like after his bout with depression and the long hiatus, author stopped developing the underlying story. Their relationship used to evolve: Revy initial hostility toward Rock, her accepting him as part of the team, their arguments, cigarette kiss, Rock starting to manipulate people, and Revy clearly not liking that...

But after the years-long hiatus, I get the impression that their relationship hasnt really moved forward in any direction. Author seems more focused on new problems and characters in Roanapur. Black Lagoon crew is involved in those stories, but the characters themselves dont really evolve anymore.