I would have the show as is through season 9. I agree, as others have mentioned, Season 10 would be the best of last three seasons to make up 22 episodes. The finale would be a two part episode for 23 and 24. During season 10 there would be three season long sub plot lines:
1. McGarrett is offered a job in Naval intelligence by the senator off and on during the season. The governor also tries to convince him to take it at times.
2. McGarrett has a love interest during the season. He meets her by stopping to respond to a minor fender bender or she is new to his Karate class. They get engaged mid season.
3. The team has outgrown the current office and construction is beginning on a new state of the art H50 station. McGarrett has a model on his table in his office and tours it at the beginning of an episode or two, is seen choosing paint colors, etc.

For episode 23, I think one of the more mediocre episodes is redone and shortened a bit to leave time at the end for McGarrett's wedding. This takes place on the stairs of the palace. Much like Jeff's suggestion of bringing everyone back for Chin's funeral, they come back for the wedding. The final few minutes of the episode is Wo Fat standing in front of a Chinese government review board where they tell him in subtitles that this is his last chance to do something about McGarrett. Episode 23 ends here as To Be Continued.
For episode 24, Wo Fat kidnaps McGarrett's new bride and Chin on the way to the reception. Wo Fat has set it up that McGarrett only has time to save one but not both. However, with everyone in town for the wedding, McGarrett comes up with a plan. Chin still dies in the last episode but not before an argument between him and Wo Fat about honor. McGarrett's bride also dies at the end and McGarrett goes after Wo Fat. The final fight scene is on a roof top (it would not be on a tram) that is full of Kung Fu, Karate, and so brutally violent that CBS gets hate mail. McGarrett in the end has Wo Fat held over the edge of the roof with the ability to drop him at any time. McGarrett then says, "For (and includes the name of every Wo Fat victim since Cocoon including Chin and his bride)". I am not sure how long this list is but Mr. Mike probably has it somewhere on the web site. Danno shows up while this is happening and yells "Steve!" every few names or so. In the end McGarrett doesn't kill Wo Fat but tells Danno to Book Em. Wo Fat's plan all along was to disgrace McGarrett by getting him to cross the line even if it meant himself, Wo Fat, has to die.

After a much needed commercial, its 1 year later. Danno is in the office with Jenny packing up the last few boxes and tells her he will meet her at the new station. On his way out he takes his name plate off the McGarrett's old office door and a picture of Chin off the wall and places them in his box. He pauses as he closes the office door one last time. On the way to his car, which is now a Mercury, he hears, "Dispatch to Williams, Dispatch to Williams. Shots have been fired at the intersection of X and X." He is also to call the governor on his way. We then cut to McGarrett at the cemetery at his wife's grave. He is in a Navy uniform and his hair is high and tight. After a tearful good bye, he goes to his beige sedan with government plates. On the radio is a disco version of the love theme. The song ends and the announcer says there was a shooting during a prisoner transport that is rumored to include a Chinese agent. The next song is a disco version of the military theme.

A few years later Jack Lord does make it to Magnum PI as a Naval Officer from the past. So much fan mail pours into CBS that they negotiate a record contract with Jack Lord to be the star of a show called NCIS. Che's character guest stars from time to time. This alternate universe presumes Jack Lord's health is good.

Hawaii Five O stays dormant until the mid to late 80s when CBS rushes a show to production simply called Five O. It is a reaction to Miami Vice and has very little to do with the original except it takes place in Hawaii. It is a flop and is cancelled after a short single season.

Hopefully I didn't jump the shark too much with my version of how to end it. Actually i should Google if the Fonz jumps the shark prior to when season 10 would have ended.