The comic series Inhuman has introduced a wide array of new Inhuman characters.
With the revelation that Skye in Agents of SHIELD is an Inhuman many of these characters could appear in the show. As these characters are not attached to the Inhuman royal family, they could be added to the show without messing up the upcoming Inhumans movie. The comic has also add multiple new Inhuman groups that could appear in the show.
Nur / Frank McGee
Dante / Inferno