What are Apparently the Rule of Crossovers?

In the world of "you can do this, you can do that" and "don't do this, don't do that" you can do what you want as long as rules aren't violated.  So there's the freedom zone protected by a restriction zone.  Now what could be the rules of mega crossovers?  As somebody who had played Super Robot Wars or have talked with its fans like Chris X, or with fans of such like Fantasy Leader I just thought of the rules based on my mistakes in Super Sentai vs. Power Rangers or other fics.  Now what's next?  As a writing critic of sorts with other fellow critics, I do hope I get them straightened up:

1.) When crossing two obviously standalone series, feel free to add non-canon events or even an alternate story.  I was afraid to do that in my Kamen Rider Kiva vs. Danny Phantom fic ignoring in Super Sentai vs. Power Rangers contains a lot of non-canon like Gai Yuki dying and going back from the dead just a few hours after he died or how the Gokaigers and PR Samurai are alternately different from their canon counterparts.  In Kamen Rider Kiva vs. Danny Phantom, obviously a lot of events don't coincide within their canon like Taiga now having his Dark Kiva powers and Mio is still alive.  In Super Sentai vs. Nickelodeon, Basco was revived as a Gedou while in Gobusters vs. Gokaiger, something else revived Basco.  In the Marvel vs. Capcom series, notice how many events there do not coincide with the respective continuity like since when did Akuma become a horseman of Apocalypse or Onslaught's attack on Capcom's world is so non-canon.

2.) As much as possible, don't try to cram in too many series when doing a Mega Crossover.  That was my mistake in Super Sentai vs. Power Rangers resulting in a crazy uncontrollable crossover.  Hmmm it does still have some readers but I hate it whenever my few readers push me to write more when I can't they should understand the writers of fan fiction are different from the writers of actual Tokusatsu.  Now looks like that goes to you don't have to put EVERY series inside your Mega Crossover.  I just had a talk with Fantasy Leader's ideal crossover.  So I decided to ask him, "So what are your favorite series?"  So for Super Sentai vs. Power Rangers, I'll avoid putting anything beyond Gokaiger while cutting down other series namely the Kalish seasons of Power Rangers to avoid going haywire.  So what I did in Super Sentai vs. Nickeloden was to tame down the characters like not every Nickelodeon show or every Super Sentai show is shown, though for Super Sentai the non-included shows have their characters still in it.  I had to play Marvel vs. Capcom series again to see where I went wrong.

3.) Keeping persons in-character.  That's been my mistake in Super Sentai vs. Power Rangers.  It does take a lot of note keeping to make sure I don't do those mistakes.  Even if there are non-canon, I could remember Chris X's advice to try reading Super Robot Wars transcripts and compare them with canon.  Very useful advice though too bad I can't understand his fic of Persona vs. Blazblue since I never got into those series.  But I wonder why he hasn't written any Marvel vs. Capcom fics himself.

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