The Five Way Race
This falls into the day-dream category, but it's worth throwing out there.
Presume the Dem race is tight, for now, and goes all the way to the convention. The GOP race goes to the convention also. Sensing weakness and opportunity, Bloomberg throws his hat in the ring with a self-financed race. The Greens mount their own challenge for watermelon hegemony. Losing the final round of convention votes (with lots of knife wounds in the back) Ron Paul utters
et tu GOP and goes 3rd Party.
What a free for all that would be.