It looks like Sonic and Mega Man are about to have it out with Mario as the referee in this piece by Mark C. So what is Pac-Man doing crashing the party? I believe he is about to take down all three of them to prove his supremacy in the video game world. You can make this battle a reality with the addition of Pac-Man to Super Smash Bros 4 (where you can also play the dog/duck from Duck Hunt).
Sadly Mr. Do! is still not a playable character.