Gothic King - Constantius and the Goths

Constantius has come along, saved the day and defeated all existing enemies of the imperial throne. But as arl was falling, a new new usurper appeared along the lower rhine. The appearance of this gothic horde is going to have a major impact on the precarious military and political balance of the northwest empire. Who will the goths side with? Who will they then betray? And what would the ramifications be of a gothic king, like, say, atolph, fathering a son with an imperial princess like,. say, galia placidia?