In Star Wars 7: The Force Awakens, Domhnall Gleeson is supposedly playing an Imperial commandant who defected from the Rebellion and is helping the Empire hunt down John Boyega's Finn. Gleeson's character is also attempting to destroy two major weapons from both the Empire and the Rebellion.
There was a rumor that this character is the son of an important character. This is what lead us to the conclusion that he is the son of Luke Skywalker. This would make total sense, especially if the rumor that parts of the script are drawn from the Expanded Universe is true, because in the EU Luke's son is slightly apprehensive about the use of the Force.
Recently I have heard another rumor. This rumor( or theory) was that Gleeson is playing Wilmuff Tarkin's son. For those of you who don't know who this is, Tarkin was the old guy who was the head commander on the Death Star in A New Hope.
This could work, however I believe that the Skywalker spawn would be much better. I highly doubt that if Tarkin had a son, they wouldn't be a rebel. I also don't think that Tarkin is that important of a character. He is in one movie and he does not have a huge role. He is not as important as Luke Skywalker.
The second reason that Gleeson's Star Wars 7 character needs to be a Skywalker is theme. There seems to be an ongoing theme, at least in the original trilogy, of fathers and their sons being on the opposite sides of the conflict. It would be perfect to have Luke's son be on the Empires side of the war, while Luke is on the Rebels side. It would mirror the relationship between Darth Vader and Luke in the original trilogy perfectly. That relationship was missing from the prequels and may have contributed to them being terrible movies.
On a different note, if Gleeson is not Skywalker's son, there still may be a father son conflict in the sequel trilogy. Rumor has it that Kylo Ren and Rey are the children of Han Solo and Princess Leia. I think it would be cool if this was the case and Gleeson was Skywalker's son so that [Star Wars: Episode VII — The Force Awakens](movie:711158) has a double mirroring on the Vader and Luke relationship. This could help strengthen the sequel trilogy in it's connection and similarity to the original trilogy.