Hanging plot thread no. 4

In The Assest an episode from the first season of Agents of S.H.E.I.L.D Ian Hart plays the role of scientist Franklin Hall is kidnapped by one time business partner and nefarious industist Ian (David Conrad) Quinn. However this turns out to be a part of plot by Hall to use a gravity field to cause mass destruction until he finds himself dropped into the gravity field by S.H.E.I.L.D agent Phil (Clark Gregg) Coulson thereby turning him into a gravity force called Graviton and put into S.H.E.I.D custody. But halfway through the season agents of the evil organisation H.Y.D.R.A take control of Graviton and place under the control of Quinn. In the season finale Quinn is seen fleeing from the scene of H.Y.D.R.A's defeat with the container that has Graviton. There has been no mention of either even though Graviton is an actual comics character in the 4 season that have followed