Clues have appeared from various sources.

There are three "ghosts" appearing in the final episode: Wo Fat, Hesse (villain from early in the show) and McGarrett's father. This suggests to me that McGarrett is going to die.

Katrina Law (Quinn) was interviewed recently. She said the ending is very, very sad, but words to the effect that "something will happen that a lot of fans have been hoping for."

According to one of my pals, Michelle Borth (McG's girl friend Catherine) was seen in Hawaii but not at anything that was filmed publicly; my friend speculated that she may have been filming on a "closed set." If true, this means that McG's GF is returning for the finale. Borth was recently in the Shazam movie and was reportedly offered a 5-movie contract with Warner Brothers, so I doubt she would be appearing in any more episodes of the show than this one.

Although a lot of fans (not including me) really hate Catherine. I suspect that Catherine will reappear and, if McG is dying, she will hold his hand.

What else could happen?

McG is told that he has to quit his job because of health complications (a parallel to Alex O'Loughlin in real life), maybe his liver transplant is failing? I dunno about this possibility. If that is the case, I'm sure that some member of the "ohana" could take over the team, though it wouldn't be the same because to most fans of the show, O'Loughlin is "the show."