Okay that is the first time that I do this, and my native language is not English, so please forgive me any translation errors. I have a great fantasy and often interesting ideas for fan fiction stories and crossovers, but these are the most crazy and/or hilarious of them, and I just thought, if I don't write them, I could at least let someone else if he wants to. Here are my three ideas. Anyone who is interested to write one of the stories is free to do so, if you want you can use any of my suggestions, some of them are needed so the story makes sense, but if you have a better idea feel free to use it.
Crossover: Haruhi Suzumiya/To aru Majutsu no Index
Story: Kyon is wondering why Koizumi and Asahina are acting strange and then he hears of the news from Taniguchi, an old friend of his is transferring to their school. Soon he finds out that it is indeed the brother of Haruhi, a person of whom he has never heard before, his name is even stranger then his own, they call him Aogami Piasu. The day he comes to the town weird things start happening, Kyon acts like a Casanova for no reason and somehow gains super pheromones, Asahina starts acting like a classical anime airhead, even calling herself stupid, Nagato for some reason starts to read romantic novels with explicit descriptions, Kyon´s sister is calling him Oni-chan and Haruhi does the same the Aogami!
Suggestions: Yes you are not high, the idea really involves, making Aogami Piasu Haruhis brother. In addition he has the same powers as her, with the exception that his are even stronger. The main reason that he was send away is because of the difference between the siblings, Haruhi has to much common sense to change things on a whim and gathers because of that her bad mood, because of that she is either a normal girl, or the destroyer of the world, her brother is another matter, there is nothing like common sense in him and because of that he uses his power unknowingly every day because of every little idea or misunderstand. So they send him to a place full of Espers, to make him bored of the supernatural, so he wouldn't spam all kind of crazy things. Unfortunately that brought him to his hobby, and now Kyon has another god around him, and this one is a hardcore otaku.
Crossover: Haruhi Suzumiya/Overlord/Disgaea
Story: Haruhi is bored, nothing interesting is happening and it seems like that won't change in the near future. After seeing a movie of the computer game club about the history of adventure games she wishes for a life like in such a game. The next morning she, together with the others wakes up in front of a certain dark looking tower. And why is there a health bar above each of them?
Suggestions: You ask how anyone can mix these universes. Pretty easy, my suggestion would be that the world of the Overlord stories is another netherworld and the reason that no other Overlords try to take the place over is because hundreds of years ago heaven and hell decided to declare the place a nature reserve. Yes it is the only place with elves, dwarfs and Halflings, not to forget many other species. Whoever wants to write this can of course decide himself if Haruhi becomes evil, but I would suggest. Instead of Evil corrupting Haruhi, let Haruhi corrupt the Evil including the minions. Maybe with the exception of Gnarl, it would be funny to see him creating a plan of making Kyon the Overlord. You could give them all their own powers, I had an idea of Kyon being a member of a long dead race of demons called Deathpan Snarker, who can create explosions by making sarcastic comments or pointing out the ridiculousness of a situation.
Crossover: To aru Majutsu no Index/Pokegirl
Story: Kamijou Touma thought it would be a normal day, until he sees a girl in space suit being abducted by anti-skill. Nobody except him is there and so he decides to help the unknown girl. Under her own protest he takes of her helmet and unknowingly brings his own misfortune and a bigger one then he has ever seen. But first he wonders why the foreign girl is cosplaying.
Suggestions: I have read a couple of Pokegirl stories, and noticed that there are virtually none with the characters from the series the main character comes from. It always goes like that, some unlucky or lucky guy somehow ends up in the Pokegirl word and starts there a new life, either as tamer or Pokegirl (in one case I have seen). In this story it would be, that a Pokegirl would be, either trough an experiment of Aleister, created or brought in our world. I would recommend the second, because then the girl could explain a couple of things. Whatever, in difference from the original Pokegirl system, here being a Pokegirl is an illness, a highly contagious one at that. Also, I always thought it was weird that Pokegirls would do whatever a tamer says and accept any treatment, even tough they can more or less kill any human if they really want to. Maybe you could give it some kind of hormonal reason, like that they can't harm, or disobey their tamer, if they try their body acts on itself. The trigger would be the taming process, and the poke ball would be in able to replicate the needed hormones, but only to a certain degree, before it is empty.