2. Make The Crossover Valuable To The Shows And Characters
The resolution of the crossover event should propel the events of the individual shows' plots and character developments. They shouldn't part ways and declare "let's never speak of this again" explicitly or implicitly. The heroes should meet each other as the mystery requires them to, and they should leave each other better prepared for their next in-show character development or plot point. Please let the crossover have a function - it makes it more worth while to watch multiple shows and try to string clues together, and makes it worthwhile to invest in. Additionally, if viewers know that there is consequence and substance to the event, they will be more likely to invest in another one; and at the end of the day, these events should bolster the ratings of all the shows involved in addition to making for great entertainment.