A crowd mode could build on this Gladiator-spoof theme the story mode seems to have. Don't even have ledges, have a contained stage where the whole point is to just keep fighting until the crowd is completely set on the better fighter. Whether he's the champ who never takes a hit or the highly damaged underdog who starts pulling out the tricks and strategy, might just come down to who comes out on top.
If you really want to keep the theme going, have the crowd decide the loser's fate at the end. Back to a trophy, or honored as a skilled fighter nonetheless. Complete with Roman Emperor style hand gestures from the Master Hand itself. Just for reference, the emperor would turn to the crowd and let them decide the fate, not usually getting involved himself unless he felt like it. A thumbs-up, was a thumbs-up. Thumb to the side meant to finish him, nowadays we use the thumbs down because it fits better with modern gesturing.
As for a lone guy yelling...It can be quite powerful in real events, been boxing for years and some of the best quotes I've ever heard came from some guy lost in the crowd. But in a game, it's just a pre-recorded reflex that would sound out of place. That and in this country there's always some idiot yelling 'Git 'er done!'.
...That being said, if some one yelled 'Freebird!' after DK's final smash I would want to shake the hand of that programmer.