Seeing credits roll does not mean the game is over. What is your favorite post game content in a game? To my knowledge this is only really a thing in rpgs and open world games, but I could be wrong and there could be some schmup with some sick postgame content.
Personally my favorite postgame stuff is in Kingdom Hearts 2 Final Mix. There is just a stupid amount to do, including a slew of secret bosses and a whole new post game area.