When it comes to writing fanfiction, here's how I would describe myself as a writer:
If writing a story is like building a house, then I am confident in my abilities as the architect and the carpenter; I can come up with a great design and build a structure that serves its function. The tasks that come after that, I'm a little uncertain on what I can do there. I'm convinced that I can do a good job with the basics (building the floor, walls, roof, doors & windows, plumbing, and electrical wiring), but I doubt I can make it seem like a real finished product (the type of floors/carpet, what color to paint the walls, what kind of appliances and furniture to place inside).
Hopefully, that analogy will make it easier for others to understand why I prefer to have a co-author work with me on many of my ideas. (My LBT/Bionicle crossover series will be my only solo project because, among many other reasons, I'm almost certain I'll never find someone who is as much of a fan of both as I am.)