It is out of character for he Atlanta Falcons to keep an undrafted free agent. It has happened a few times in recent years, and those players typically turn into contributors. Many expected a big improvement from Stone with the move to the new blocking scheme, and he had some decent play.
The problem was he failed to earn more than 137 snaps, filling in at the center position for a few games. Stone has spent some time at both center and guard, and is under contract in 2016 for only $600,000. That price should make Stone a lock for a roster spot. Another offseason of development should make Stone valuable depth, and give him a shot at one of any three weak interior linemen positions.
Atlanta is expected to address the interior offensive line in both free agency and the draft, but that will unlikely not impact Stone. Starter Chris Chester is not under contract, Mike Person struggled at all the worst times, and Andy Levitre has struggled to block without earning a dozen flags. If Atlanta makes cuts to the offensive line, the more expensive veterans are the likely targets.
Stone should return, assuming Atlanta does not make multiple additions to the interior line this offseason. Personally, I would like to see him get another shot at center, and push Person to guard. Any later free agency signings or late round picks would likely compete with Stone for a starting spot, and not necessarily a roster spot.