I see two ways this could go. If everything was resolved, then there would be nothing left to tell, but on the other hand, if many strands were still hanging, then a future project could pick them up - be it a novel series, a movie, a mini-series, a web show, mobisodes, a computer game, etc.
The Missing Pieces were a good opportunity to tie up some of those strands, but also of course brought up even more questions.

I also liked the idea behind the various ARGs adding backstory to some non-crucial aspects of the story, such as the LOST Experience (which was all about what happened to the Hanso Foundation and the old DHARMA Initative).
I think the crucial thing is to make sure that the hanging threads are not ones vital to the important storylines, and of course this implies that we need to identify what the main storylines of importance are. For example, I and maybe several others had been hoping that the story of Libby might be fully explained, including how she ended up in the Santa Rosa MHI. Maybe it will never be explained because the creators don't feel it's as important a story thread as, say, the story of Widmore. Maybe it will be picked up in season 6. I don't know.