While the Mad Titan has already revealed himself to pose a threat to the Guardians in future Guardians of the Galaxy movies, his son could also cause issues now and possibly link up to a new series.
A villain that could show up in the sequel to Guardians of the Galaxy and one name connected directly with one of the villains from the previous film is Thane.
In the comics, Thane is the secret Inhuman son of Thanos. After a descendant Inhuman tribe ran afoul of Thanos and his army, one Inhuman woman returned home pregnant with Thanos' child. Years later, Thanos started searching for his lost son, disguising his quest as a new mission of galactic conquest.
He and his Black Order would raze planets, demanding a tribute of the heads of younglings between the ages of 16 and 22 (the estimated age of his lost son), or the annihilation of its inhabitants, his search eventually leading him to Earth.
When Thanos fought Black Bolt in Attilan, the Inhuman king detonated a Terrigen Bomb, which subjected any person with Inhuman genes on the surface of the whole Earth to Terrigenesis. Thane's powers were unleashed, and killed every inhabitant of his town in the process.
Ebony Maw, one of Thanos' lieutenants, arrived to seemingly aid Thane, and provided him a suit to control his powers. Maw also trapped Thane in a containment field, and informed Thanos about the finding of his son.
Thane would allow for another descendent of Thanos to join the field after Gamora and Nebula. The Inhumans were also mentioned in his origin, which is a property that Marvel is now interested in marking with the possible aid of Vin Diesel.
Thane could be introduced as a side villain and lead into the Inhumans movie, in a role similar to Nebula in the first movie.