If you want to start your writing career off with fanfiction (which is what I did, so you're in the same boat as many people), my best advice would be this (and it's less a bit of advice than it is an imperative, one which you're in a unique position to be able to do as a BIONICLE fan):
Create your own characters!
I would stress this more than anything else. As a writer, you're going to have to move onto your own projects eventually (and you're obviously hoping to), and with that in mind, writing characters whose personalities and histories are already created for you may be easier, but it forestalls the essential skill you want to cultivate--your own creativity.
That's not to say that you can't use preexisting BIONICLE characters in your story. You certainly can. But getting inside the head of a character who already exists is one thing. Creating characters and fleshing them out on your own is another. Even if you use the building blocks BIONICLE has given you, like masks and powers and Elemental personalities, creating your own characters will automatically elevate your writing beyond standard fanfiction.
As for ideas, those suggested thus far are good. I would recommend taking one of them and developing it into something unique to yourself. Think of the stories you've read and loved, and take elements from them. An example, perhaps:
Three of my favorite stories are The Lord of the Rings, Star Wars, and Avatar: The Last Airbender. Say that I wanted to combine elements that I like from each of them. I like the idea of a magical object at the center of a quest that comes from LOTR, I like the idea of an order of knights that protects the universe from SW, and I like the idea of a reincarnated, deity-like figure from ATLA. How could I combine those into a story? I could reinterpret the idea of the One Ring as a relic that the order of knights is tasked with protecting, and perhaps the reincarnated figure is their leader. At this point, I can ask myself questions. Why is the leader of this group reincarnated? Maybe they are some kind of ancient spirit who founded the order. Maybe their life-force is tied to the relic. Maybe the reincarnated figure is a slave of some kind, bound to the relic by the knights, who are nefarious, and need to be defeated by the main character.
This kind of combination and repurposing of elements that you already know is where I think you can start crafting a good story. You can start with characters, or you can start with the world they inhabit, but my suggestion would be to start with a conflict. What drives your story? What is the goal, and how then can you fit in characters and concepts into that goal?
I hope this advice is helpful! Good luck beginning your journey through the fabulous world of storytelling, and keep us posted on your progress. I'd love to see what you come up with!