Since no one seemed to mine my takes on the last two Highlander sequels, why not continue?
As far as the fourth (Highlander: Endgame) goes, I'm not sure I'd change too much. The only plot point I'd alter is the assault on Sanctuary. It would still be on holy ground, the catch is Kell (Bruce Payne) has his Immortals attack the Watcher guards, while he uses regular mortal mooks to kill the Immortals.
But what about Highlander: The Source?
It's in the far future. There hasn't been a new Immortal in decades. By this point, you fit the all the Immortals on Earth in a decent sized SUV. So, what about the Game
Duncan MacLeod (Adrian Paul) has the idea that they should scatter. Just live as they do on different continents. Decrease the odds of them decapitating other at least.
And this works, at least until the cosmic alignment. A strange figure calling itself the Guardian arrives. He's faster, stronger, and generally meaner than anyone else, as evidenced by his getting some Quickenings not by fighting with a sword but by using his bare hands. He makes it clear he is coming for MacLeod and he'll slaughter anyone who gets in his way. He proves this by taking out most of the Watchers.
Anna (Thekla Reuten) is one of the few Watchers left. She took over for Joe when he retired. The Guardian seems to driving them somewhere. And what is he guarding?
The Source-what makes Immortals immortal. By not finishing the Game, the surviving Immortals are at risk for upsetting the cosmic balance. The Guardian's task is to make sure the Game is finished on time. Duncan isn't keen on killing his friends. Methos forces his hand by fighting the Guardian. He loses, but he gives up a good fight.
At this point Duncan is the last one left. The Guardian directs him to the North Pole for the last fight. Since the Guardian has all the Quickenings, he argues, it will be up to Duncan to kill him. But since the Guardian wasn't a proper Immortal, Duncan fights and manages to kill him. Duncan rejects the Prize though. sending the Quickening over the Earth to restore and renew the land and people.
Also, the only dialogue the Guardian has would be Queen lyrics, because why not?