For a while scholars like Robert Eisler (1930s) and others had entertained the idea that some of the unique stuff to be found in the Slavonic version of the War but not in the Greek, may be due to the influence of an Aramaic work Josephus created to dissuade the Jews of Mesopotamia, and I suppose their Parthian rulers, from helping revive the vanquished rebellion in Judea. No trace of the original Aramaic has survived. We only know it probably described in vivid and probably gory detail how the Romans reduced the Judean strongholds in Galilee and Judea, especially Jerusalem. However, because it served as the basis for the Greek War, it may have also provided some background to how the situation degraded so far, but again we are just guessing. Perhaps Unterbrink thinks some of it is relevant.
However, two researchers (Leeming & Leeming), had since proceeded to compare accounts of things found in common in the War, Antiquities and Josephus' own Autobiography, AND the Slavonic translation of the War, and concluded that the more likely source for the variant accounts (Slavonic War versus the Greek War, Ant & Life) was semi-pagan feudal lords in the Russias, who liked the War for its descriptions of battle tactics and the politics that drove them on. They speculated their own speculations and, since the Slavonic War is not really a translation but a paraphrase, they wrote them into the narrative or left out the boring parts (to them). So, no room was found for an Aramaic version for the War.
One of our forum members (MH), who has a copy of Leeming & Leeming's book, still advocates for Slavonic War because she feels it supports her belief that the last Hasmonean royal claimant, Antigonus the son of Aristobulus II, was the figure upon which the NT authors based their portrait of Jesus Christ.
DCH