Well, there is quite a bit of fan-fiction on the 'net about Rose and Jack. It's not legal to use the characters because it infringes on the author's or authors' copyright, but it's there. I've read some of it and, like all stories, some are well written, some are ok, and some are poor and would not have found a publisher even if the names were changed.
Some would be another's cup of hot chocolate but not mine.
IMO: There is always a market for a 'Titanic' story, but anyone who sent one about Rose and Jack for publication would be disappointed, or would probably need a clever copyright lawyer.
I think anyone wanting to legally do an adaptation of James Cameron's Titanic or any spin offs based on the story would have to start with Paramount to get the needed permissions as they're the ones who own the copyright. It may well be that Fox (Which partnered with Paramount) and James Cameron would have to sign off on this as well.