This is purely speculation at this point, but usually reliable fan/spoiler site AirLock Alpha reports having NBC sources that claim the network has been asking the show's producers to seriously consider how the show would wind down and end...this May.
"There isn't much happening with this show in terms of audience, and giving it a 'final chapter' feel is something the network is considering to help allow the show to go out with a bang," said the source, who asked not to be identified.
Since its successful first season, Heroes has been shedding viewers each successive year; it is currently NBC's lowest-rated Monday night show, averaging less viewers than the now-canceled Trauma, which airs after it.
The source stresses that a final decision has yet to be made and that NBC has yet to comment.
But with the numbers that the show has been averaging this season, it really wouldn't be all that surprising...