Fanfiction.net is also a great place. There are many talented writers who post their works there. However, the quality can vary, so you might need to do some sifting. Look for stories with high ratings and positive reviews. For Danny Phantom and Blue Exorcist crossovers, try searching for specific keywords like 'Danny Phantom Blue Exorcist crossover' to narrow down the results.
Character development is also a big theme. Writers might take a character from one series and put them in a situation from the other series to see how they grow and change. For example, Danny might find himself in the world of Blue Exorcist and have to learn how to deal with the exorcist's way of fighting demons, which could lead to new skills and a deeper understanding of his own powers.
Yes, the Blue Exorcist manga is over. It took readers on an exciting journey and concluded in a way that left a lasting impression. The final chapters brought closure to the characters and their adventures.