It depends. Suitably epic plots sometimes get the honor of having a whole world designed around the Grand Plot. Other times, a published setting (FR) is default unless the DM decides not to. Normally, published realms are never effected by our heroes actions in a previous campaign, because we tend to play huge, overlying plots that would be silly to keep including. ("Ok, so we finished last campaign with you overthrowing Durbin and stopping him from taking control of the eastern half of Faerun. Next campaign, the Zhents will try to summon demons to take over Cormyr...")
Right now, i'm working on a setting that hopefully will be the norm to RP in, but its the players choice.