A random idea, but very different angle on the identity of Brahmas came to me today.
Perhaps the Brahmas were never an actual species, instead being the conglomerated product of the two entirely different Shivan and Vishnan species. The Shivans know only to destroy, the Vishnans know only to preserve. These two properties together however allow for creation which in itself is the act of scrapping concepts deemed not useful (destruction) and keeping concepts which deserve merit and further consideration (preservation). To explain why both the Vishnans and Shivans believe the Brahmas to have been a seperate species, one must look at the nature of these two ancient and surviving species. To the Vishnans the act of exacting destruction, and to the Shivans the thought of maintaining preservation are SO alien, that neither can comprehend nor grasp the concept of creation which requires an understanding of both destruction AND preservation. Thus so the act of creation was never understood by the Shivans and Vishnans, it was merely the product of a successful collaboration between the Shivans and Vishnans and for them it was convenient: life kept developing and both could keep fulfilling their respective purposes. Somewhere in the course of time things went wrong and their alliance faltered. The truth behind the true face of creation became corrupted and misinterpreted and the workings of their alliance ceased to be. The alliance was now a mere officiality: both the Vishnans and Shivans knew the alliance was apparently important but neither knew why. Ironically having a narrow and biased perspective is the one and only thing the two share in common and so they both agreed there must have been a third species which was capable of creation: a concept they never have or will understand. Because these mythical 'creators' are nowhere to be seen, they are assumed to have disappeared a long time ago and are considered to have perhaps perished. There never was a Triumvirate of Shivans, Vishnans and Brahmas. It was a Duumvirate, an alliance between two species, two species which despite being eachothers nemesis formed a mutualistic bond through which all in excistence could prosper in the long term. Two species which forgot they are two sides of an automaton which must not deviate from their purpose. Humankind or Vasudankind aren't the Brahmas, nor will they be. If they prove themselves worthy, they are tolerated for the time being until becoming malevolent to the universe. But MAYBE if the Terrans and Vasudans can solve their conflicts, the Vishnans and Shivans for a brief moment will observe and realize the miracle of creation, the value of an alliance. Indeed brief, but clear and evident enough to realize the importance of their roles: destruction and preservation, again in perfect balance to resume their part in creation again for the next few billion years.