Dave Meltzer noted that a star about to become a free agent that left WWE and would "make a lot of money" upon a return is welcomed back by WWE's front office and there appears to be a mutual interest. However, this person supposedly didn't like WWE during his first run.
There are two people that immediately spring to mind and fit the perimeters: Bobby Lashley and Jeff Hardy. Lashley's contract status with Belator is unknown, but his contract with TNA is probably coming up. Jeff Hardy's contract is up in January.