Should the NHL decide to expand to Atlanta, this would be third time they would've been granted an NHL franchise. The Atlanta Flames were the first NHL team awarded to Atlanta and were active from 1972 to 1980 when they relocated to Calgary. The NHL returned to Atlanta for the start of the 1999-2000 season with the Atlanta Thrashers. The Thrashers did have some talented players like Danny Heatley, Marian Hossa, Ilya Kovalchuk, but only made the playoffs once, losing in the first round during their second tenure in Atlanta before relocating to Winnipeg.
While Houston never had an NHL team, they did have the Houston Aeros that were part of the International Hockey League (IHL) and American Hockey League (AHL). Beginning in 1993 in the IHL, the Aeros were a successful IHL franchise, until the NHL began supporting the AHL and told NHL teams to begin using AHL teams as their affiliates. the Aeros became the first AHL affiliate of the Minnesota Wild and then became an affiliate of the Dallas Stars until team relocated to Iowa due to a rent dispute with their arena owner.
As the rumours of NHL expansion are recent, there is no confirmation that either the Aeros or Thrashers would be revitalized should Houston and/or Atlanta be awarded NHL teams and it should be noted that there is no discussion of either of these teams being relocations either. Although many would agree that Arizona should be relocated and should have been relocated years ago, the fees for teams to enter the NHL are too valuable to be passed up by the league.
Personally, I think the NHL could benefit from having another NHL team in the greater-Toronto area. With the Leafs being one of the 2 most profitable teams in the NHL, having another team that is almost guaranteed to profit could do a lot to help boost the salary cap.