Two big things:
First you need to have characters whose universes can be meshed together. If you have Ben, you have to explain how the aliens from KP or Jenny worlds work. If you have Jake, you have to think about how the Phantom Zone works togther with the rest of the magic community, and how much the dragons know about Charmcaster or the Monkey Master.
Second, you need a series focused on the crossover. A few initial cameos or one character appearing in the others comic/show is good to establish a shared universe, but you need a team book to explore character dinamycs, establish how the universe works together, etc.
Both the Justice League and the Avengers are basically crossover books.