The 2002-2003 Dallas Mavericks are one of the single greatest "what if's" in modern NBA history. If not for a sprained MCL in Game 3 for Dirk Nowitzki, Dallas likely slays the eventual champions and then makes quick work of the New Jersey Nets in the Finals to claim their first title.
A championship for the Filthy, Dirty, Nasty Mavs means Steve likely never leaves for Phoenix, and his MVP-caliber basketball is lived out in Dallas. It also means the now second-winningest coach in NBA history, Don Nelson, gets himself a title and redefines his legacy. Who knows what would've followed had this great "what if" gone just a little differently.