I think the major problem is:
1. The original creator is not involved, so, like all fan fic how close are you going to be?
2. Do you know BOTH IPs to do a cross over well?
I started doing a fanfic involving World of Reverse Morality. I AVOIDED the original character as much as was physically possible. We don't even MEET one until book 2. Even then, my version are different from the original in that, I have made assumptions. I assume that the ML and FL both dislike each other. I have made assumptions about motivations, but I have tried, SO HARD to at least work with what I was given.
THAT IS THE FUN PART.
And we are only talking, what? 32 'episodes' of Reverse morality? Yes, I delved into the previous version a bit, but only for background.
Now, to imagine doing Dragon Ball Z into Reverse Morality World, or... vice versa?
Batman in 40k Nostromos
Or the Nighthaunter 40k into Gotham.
Death Korp of Krieg invading My Little Pony. (Or Black Templars vrs Furries. Been done BTW)
See, some combos are a one off gag. Some would be an interesting combo. Nighthaunter vs Batman (Would have to be in gotham Batman would SO NEED the hometown advantage vrs the Nighthaunter.) But most... well... lets face fact, what sort of staying power does that have?
Most stories do not have ROOM for more than one MC. You add another MC from another IP, what you get? SOMEONE HAS TO BECOME THE ANTAGONIST. Or, you have to set it up where they were intended to come together.
My three stories are written to come together. I planned it. They are a prequel to when these guys come together. I have two other stories to write so they become a 5 man band. They are all written with different Genre, but have enough in common so the crossover will make sense. EVEN THEN, the final product will be a tad-bit anthology.
I like anthologies. I should post up my Tales from the trenches.
My point is, is sounds cool. Take awesome. Add another awesome. you should ave awesome squared right?
No.
Take a birthday cake and put it in a blender with savory beef stew. Is that awesome?
THAT IS WHAT MOST FANFIC IP CROSSOVERS ARE.