This is a topic I've been considering for quite a while. As it normally stands in video games, you smash your way to the end to come face to face with a final boss. When that time comes, the final boss most often has special music accompanying the fight. I'm trying to figure out how many games play with this idea and play the hero's theme instead of playing a standard final boss theme, or otherwise play with the idea of final boss music being used elsewhere in the game. So, yes, this thread will have plenty of spoilers.
Some of the most memorable examples I can remember include the following:
Double Dragon's final boss having the title screen music, which is often titled "Roar of The Double Dragons" if I am not mistaken.
Sonic Adventure plays "Open Your Heart" during the first half of the Perfect Chaos battle, which is not technically Sonic's theme but does play during the title screen demo.
Sonic Colors has a remixed version of Reach For The Stars playing during the second phase of the final boss, which is also the main theme to the game during the intro.
Dragon Force's final battle has the title screen music playing as you fight Madruk.
If anyone can think of anything else, I'd like to hear it.