The opening sequence is something about how "for centuries, dogs and cats have been natural enemies" and the visuals will include scenes from history, like Ancient Egypt, and from pop culture or something, and, well, some intro to the game before you pick a team.
The first opening levels will take place in a city. They both have secret headquarters, but the dogs have an underground place with peanut butter and toys and a place to run around, and the cats pretty much have a secret society. (This game has people, too, but this storyboard is laggy enough as it is.)
Some characters travel from alternate dimensions to help out on Earth. These include people and animals from the past and future, gods and goddesses from mythology, and all kinds of creatures and entities from worlds other than our own. As it turns out, the cats chose this as a good time to invade because transdimensional rifts, sealed away for centuries, were starting to open back up.
These other characters have their own unique abilities (though it turns out later that the city ones did too), and are powerful, and add a lot more to the story. Fighting alongside Hades and Cerberus is easy at first until you realize that Nyan Cat is blasting rainbows everywhere and the Easter Bunny has exploding eggs. Both sides leave the city, are joined by wild animals along the way, and defend or take over more prominent areas.
For the final boss battle, everything comes down to this one moment, and all of the most powerful characters are there. (They look weaker in this picture, but in the game they are fully upgraded, and by now either have magic, weapons, or a very strong bite.) This won't all be based on fighting; there will be some other aspects involved.
The sequence will change depending on which team the user was on. Either the dogs are heroes and everything goes back to normal (aside from those aliens over there), or we live in a dystopian future ruled by cats. Other actions can take place past this point. For example, the cats can have a change of heart, and both endings will have to deal with the occasional uprising.