Originally, I was going to write about why King Mickey should die in Kingdom Hearts 3, but then I realized Disney would probably never let that happen, so then I thought about the one character that would have the greatest effect on the Seven Guardians of Light: Riku.
Why It Could Happen
Riku is one of my favorite video game characters of all time, let alone in Kingdom Hearts, his development has been extraordinary over the last 14 years, and he is one of the most powerful keyblade masters (as of Dream Drop Distance) ever. His ability to wield powers of both light and darkness is unparalleled by any of the other (confirmed) members of the Guardians of Light. His death would deeply affect Sora, Kairi, and The King- half of the seven. Of course you also have to remember that Donald and Goofy aren't part of the seven, and his death would still deeply impact them as well. His death could create a massive turning point in the battle against Xehanort and the 13 Darknesses.
How It Should Happen
If there's anything we've learned throughout all of the Sora-centric games in the series, it's that Sora can fend for himself. He may not be a master, but he is still a very skilled keyblade wielder. That said, I do not want to see Riku die for Sora in Kingdom Hearts 3. My nightmare of a death scene is Riku jumping in the way of a keyblade slash for Sora (I cringed as I wrote that). Both of these characters, as well as all of the other guardians, save for Kairi and Axel (Lea?), are incredibly skilled wielders and masters who can easily fend for themselves. By the time the final battle comes, throughout the game Sora and pals will likely have been fighting through the 13 Darknesses as well, like the Organization in Kingdom Hearts 2. When final battle comes in Kingdom Hearts 3, Sora and Mickey will probably fight Xehanort, but when the Traitor Foreteller (you can read why I think he'll be there here) shows up, Riku will go fight him. However, Riku won't be as lucky as Mickey and Sora, unlike them who are almost guaranteed to win, Riku will lose, and as a result, die. This death will cause both Sora and Mickey to abandon the fight, and instead run to Riku, while the Foreteller escapes.
How It Changes The Series
Obviously, this is all just a theory, so nothing is confirmed, but the only character confirmed for after Kingdom Hearts 3 is Sora, who will return as the main character. If Riku does in fact die, everything Sora does goes from trying to find Riku, to being in his memory. Not to mention the effect it would have on Mickey, who is exceptionally close to Riku, having gone through the realm of darkness with him. The next few games would change from searching for friends, to avenging them if he does die.