It's ranking time, well this time it's more of a list. When it comes to power systems in OP, devil fruits are not the only powers to be considered. Below are the list of powers and abilities that are not limited to devil fruits.
1. Voice of All Things
An extremely rare ability that allows one to here and communicate with inanimate objects and animals. It's shown to be similar as Poseidon which allows the mermaid princess Shirahoshi to command the Sea Kings. Much is yet to know about this mysterious power.
2. Conquerors Haki
The rarest form of haki. It is said that only those who possess the quality of a king has it, meaning one must be born with it. This form of haki gives the user the ability to exert their willpower to dominate others often knocking them out. In it's advance form it can even affect the surroundings.
3. Future Prediction
Not to mistake with the Observation Future Sight. This abilities of Madam Shyarly can predict the events of distant future at 100% accuracy, as shown when she predicted the birth of the mermaid princess in 10 years and the coming up of the Great Age of Pirates at a young age. Although she can't predict the exact time of the happenings.
4. Ships Spirit
Well this one's not really a power more of an occurrence but it's so mysterious that Oda had only mentioned it few times. Klabautermann is a type of spirit that dwells within ships and only apears when the ship is about to meet its doom. It's said that the human form of Klabautermann only reveals itself to someone that truly cares about their ship.
5. Nightmare Luffy
Deriving from Moria's Kage kage no Mi, Nightmare Luffy is a product of 100 shadows entering Luffy's body. In this form Luffy far exceeds the power of Gear 2 and 3, boosting his physical power into new heights. Luffy also gets the abilities of the person's shadow he consumed. Luffy in this form can only last for 10 mins.
6. Raid Suits
When superhuman strength and feats are seen to be common, then the Raid Suits elevates it to beyond human realm. These technologically advanced outfits are designed for tough combat situations. Made in Germa Kingdom and donned by it's royal family the Vinsmokes, the suit gives them enhanced speed, power and durability together with unique abilities that it comes with.
7. Sulong
The Sulong is transformation gone through by the Minks into there feral state, looking more animalistic and there body surrounded by sparks of Electro. The Minks are said to be natural born fighters, after going through the transformation there skills and natural strength are increased significantly.
8. Asura
The manifestation of Zoro's spirit resembling of a demonic god the Asura, thus giving him the epithet of 'King of Hell'. When Zoro is in this state he has access to the Nine Sword Style, which increases his cutting power by threefold.
9. King Punch
I've to add this one on the list. The King Punch is essentially an one shot kill technique that Elizabello uses as his signature move. Said to even knock out an Yonko if caught directly, but it's set up which requires an hour to ready makes it impractical. King Punch>King Kong Gun in terms of pure power. Fight Me😅
10. Hakuba
This is another bizzare abilities we've seen in the show. Hakuba is an inner personality of Cavendish, as he suffers from split personality disorder. When Hakuba takes dominant he becomes ruthless and coldblooded therefore increasing his physical capabilities. Hakuba's main strength lies in his speed, where he was so fast that even with Haki he could barely be seen.
Reminder: This list is not ranked by which power is stronger, it's purely based on the rarity and uniqueness of the abilities.