Dark Spirit from Beyond, Kyoreya
The Boss Room could be a really dark shrine that worships the boss. What you would to to trigger the boss fight is use the Sacred Lantern item you aquired in the dungeon and find her at the shrine. The battle will be similar to Bongo Bongo from Oot, but she won't be invisible, just shrouded in the darkness. The monster itself will look like a mix of Gomess from MM and that Gomess-like miniboss in the Arbiter's Grounds from TP. You have to shine the light on her, she will then be temporarily paralyzed, and will give you a chance to attack her. The Ending Blow signal will finish him off.

Dragon King of the Dead, Serpenfos
This'll be like the Volvagia battle from OoT, where you use the Stone Hammer to defeat him. His lair is in an abnormally large graveyard. When you approach the central tombstone (bigger than the other tombstones), a skeletal dragon will rise from the grave and the boss battle will ensue. He will first issue a command raising all the Stalfos from the dead and attacking you. Dispatch then and he will swoop down on you. Use the Stone Hammer to knock spinal columns off. He will call on more Stalfos and additionally, four to five Redeads every time you knock a spinal column off. Once all the spinal columns have been knocked off (10 times), his head will remain, and you will have to perform the Ending Blow to finish the job.

Ancient Knight of the Deep, Aquamass
The "arena" for this fight is similar to Gyorg's lair in MM. However, there are hungry, unbiased sharks in the water. The knight is sort of like a seaweed-covered Darknut, but much larger and with different tactics. This boss has you utilising the sword-to-sword combat system seen in the fight with Ganondorf in TP. But this time it's a must, as you use this fighting method to push him off the platform. Once he hits the water three times, the sharks will devour him, and he will be defeated.

These are a few of my eight boss battle ideas... But I don't like typing too much so I'll leave it like this for now...