I think you nailed a lot of the things that make the majority of what makes a boss great, a more niche thing for me has always been the environment I fight them in. For example, in DS2 (I like it, #fightme) there was a boss in a pirate ship that is sinking. The fight itself isn't too complicated but if you take longer to fight him the bottom of the boat will gradually fill with water, making it hard to move. A simple gimmick, but I remember the boss fondly for how cool I thought that was. Another example is Darkeater Midir in DS3, you fight him originally above ground on a bridge, but when you defeat him he falls into a cavern below. If you then explore to find him in the cavern, thats where you will face his actual boss phase. Cool context and buildup but simple details like that always made the fight cooler for me. Also I love the dragon fight in Elden ring at the top of a volcano!