(copied over from SB's Kamen Rider fic ideas thread, after yet another "wouldn't it be cool if X appeared in Y?" post)
The most important question when you're talking about adding any crossover character in any setting is, "what can they do that the original cast did not or could not?" Just adding someone for the sake of them side by side being awesome tends to peter out shortly after the actual meeting, or else turn into a rehash of canon with more quips and the moments of awesome spread between more characters. Figure out what you want to have someone extra do in whichever setting you want to work with then figure out what crossover character would best fit that role, is my suggestion. Backwards from the usual method maybe, but it gets the hard question out of the way first so figuring out which favorite other character you can have shake hands (or cross swords, whatever) with canon characters doesn't have to be a guilty pleasure.
The most important question when you're talking about adding any crossover character in any setting is, "what can they do that the original cast did not or could not?" Just adding someone for the sake of them side by side being awesome tends to peter out shortly after the actual meeting, or else turn into a rehash of canon with more quips and the moments of awesome spread between more characters. Figure out what you want to have someone extra do in whichever setting you want to work with then figure out what crossover character would best fit that role, is my suggestion. Backwards from the usual method maybe, but it gets the hard question out of the way first so figuring out which favorite other character you can have shake hands (or cross swords, whatever) with canon characters doesn't have to be a guilty pleasure.