I prefer crossovers where two characters meeting would create a conflict first, it's much more interesting. Of course they can always become friends later, after they understand each other.
I have a ton of crossover fanfic ideas that I will never write because I'm not a very good writer. Here are a few:
-Swat Kats and Talespin: T-Bone and Baloo would butt heads first, each claiming to be the best pilot, but would end up respecting each other in the end; Razor and Wildcat would talk tech together, and finally, Callie and Rebecca would get along fabulously.
- Talespin and Porco Rosso: Just thought of that one. I totally see Porco and Baloo swapping pirate hunting stories while having a drink at Louie's or Gina's place.
- Justice League and Hellboy: HB and Batman met once in the comic, they get along okay. I imagine HB and Lobo being the best of pals having a couple of beers together. And Abe and Martian Manhunter would get along great too.
- Superman, Tintin and Astro Boy: Bet you're all going "huh?" on this one, but let me explain. I imagined these three together because they each are the most famous comic characters from their region of the world (Superman from America, Tintin from Europe and Astro from Japan). They're three icons of the comic world and I would love to see them together.
They also have similar personalities, all three can be qualified as "Boy-Scouts", do-gooders who are not afraid to fight for peace and justice. Now I need to find a plot that would get them all together...