Defender of the Crown did get a remake years ago IIRC.
Skimming through the list of games on Wikipedia, these ones stand out (and are far too numerous to link):
Creatures and Creatures II: Torture Trouble
Mayhem in Monsterland
The Last Ninja and its sequels
The Monty's (WanteD: Monty Mole, Monty on the Run, Auf Wiedersehen Monty)
Barbarian: The Ultimate Warrior
The Bard's Tale
Cauldron II: The Pumpkin Strikes Back
Ghosts 'n Goblins
The Hitchhiker's Guide to the Galaxy
Zak McKraken and the Alien Mindbenders
International Karate +
Myth: History in the Making
Nemesis: The Warlock
The New Zealand Story
Spy vs. Spy
Turrican and Turrican II
The Trap Door
Thing on a Spring
I know many of those are not exclusive to the C64, but that's where I played them.
Whether or not any of these deserve a remake, well, that's something for later discussion.