Mkay so I had this idea while taking a shower. Basically, you choose no less than 2, no more than 5 (unless you want 6) anime/manga, and combine them all into modern society. (i.e. President Obama is the Pokemon League Champion, but he's also a Soul Reaper, and his Zanpakutō is Kokujin Daitōryō, black president ;P)

Here's mine:
1. Digimon
2. Gundam
3. Medabots
4. Megaman

So the Digital World was created through the thoughts, feelings, dreams, and myths across the internet giving "life" to the Digimon. Once the more powerful Digimon realized that they are all nothing but data, they resolved to come to our world to become real. And so, through ways I can't even begin to think of (cuz I have no idea how xD) the Digimon began to materialize into our world. Many of the Digimon attacked human cities, mostly in self-defense, for a lot of people would attack the Digimon out of fear. From the escalating battles that ensued, the United Nations finalized that the Digimon should either be eradicated or pushed back to their own world. Scientists and robotic engineers developed the Medabot, robotic humanoids fully equipped for battle, with guns, missles, radars and such. The Medabots were distributed to all major countries, and those not able to obtain them were protected by those that did. Some countries however (I don't wanna make this a political thing so I'm not gonna name countries) used the Medabots not only to defend themselves from the Digimon, but to battle against other countries as well. Because of the immense size of some of the Digimon, larger scale robots were developed named Gundams, which would be piloted by people instead of controled by an A.I., because sometimes human intuition is better than computer statistics in battle. And, like the Medabots, the Gundams were also used to fight other countries as well as the Digimon. World War 3 began. Some scientists, wishing to at the very least stop the Digimon threat, created a computer program named Megaman, and uploaded him into the Digital World. His objective was to make it to the core of the Digital World and shut it down, which would theoretically make all the Digimon in the real world dematerialize.

*stops there*
So there ya go. Not that great but it gets the ball rolling I guess? Try to use your imagination and have fun with it~