There are a number of free agents hitting the marker: LBJ, Wade, Bosh, Melo, Tm Duncan, Dirk Nowitzki all have early termination options.
Unrestricted free agents who could have a huge impact include: P. Gasol, Gortat, Z-Bo, Deng, Shawn Marion, Paul Pierce, Kyle Lowry, Lance Stpehenson, Spencer Haawes, Ray Allen, Trevor Ariza, Vince Carter, Andray Blatche (ET), Chris Kaman, Kris Humphires, Jordan Hill.
And there are a host of restricted free agents (see here for a comprehensive list: http://hoopshype.com/free_agency_2014.htm
It is likely that those with early termination options will stay with their current teams (unless something drastic happens in Miami, in which case everything would be up in the air). But assuming that Melo is the only franchise level player who leaves, which of the remaining players is most likely to change the NBA landscape?
My vote goes with Gasol. He still seems to be playing at a high level, and on a contender like Miami or OKC, he would have the burden of a heavy scoring load lifted, but give such teams a HUGE weapon in the post that can rebound well and has championship experience, and given his age, it is unlikely that many teams would offer him a long term deal with a significant annual salary.
Gortat will likely earn a big pay day and stay with Washington, but if he should bolt, he could have a big impact on who wins next year depending on his location