The Ar 66 had a straight frontal strut undercarriage, a Argus engine, an advanced seat layout compared to the "mistery" aircraft and the lower wing wasn`t as backwards as the one in the carrier-borne biplane. The Ar 66 had a tipycal Arado fin. The Go 145 doesn`t match either. They also are too nimble.
Digging for more I came up with this: the He 45 was used as a testbed for new engines like the Argus As12, the Daimler-Benz DB600 ( November 1935, V-10 and V-11 prototypes, proposed as the first of the He 45G series), the BMW 115 and 116, as well as the Jumo 211A ( August 1936, V-12 and V-14 experimental aircraft). The DB 600C and Jumo 211A engined aircraft would afterwards be known as the He 245 A and B series, respectivelly. Info taken from Volker Koos.
I now believe the mistery has been solved. It is the He 45 or 245, with a liquid-cooled inverted Vee 12 cylinder engine. But it represents another proposal (unknown) added to the aircraft complement of the "Graf Zeppelin" carrier. This keeps growing.