Just out of curiosity, what inspirers you to write crossovers? For me, when I watch one show, and become interested it, and its characters I get a strange urge to see it combined with other shows in which I'm interested. There are many shows where I see similarities between characters, and get overly curious to see what would happen if they were to meet. I will admit that I have issues with loads of bunnies that don't get written, but I tend to prefer reading fic to writing fic. On the rare occasion where I do write, I tend to keep them short in order to avoid penning a monster, which may get abandoned before the actual fic is finished.
However, I do understand that this may not seem to be the case with other writers. I've seen a number of authors who write fics based on challenges. Nothing against fic challenges, but I tend to find that if they get too specific, instead of the challenges they were meant as, they turn into outline writing requests. If one is going to post a "challenge," keep it simple, and see how different authors take on the idea. I must say I love the way several different authors have handled the idea of Doctor Who meets Life on Mars in this community. Yes, I will get to reviewing those fics.
Another type of challenge is the "table challenge," while these attempts by authors can produce a few intestine fics, I've seen a number of authors who attempt these, and eventually in pursuit of filling their quota of 100 different types of fics, some tend to start writing "table fics" which tend to be a bit and uninspired.