Is it possible to add the TAU in DoW Elite possibly using the models from the Destroyer MoD?
Elite mod developers had that in mind when they started in 2013 but as time went it became clear that balance issues was harder to fix than originally anticipated. Grey Knights the new faction has been subject to very violent balance shifts and it discouraged developers from initiating any new factions let alone heroes. There were several occasions when adding a new Slaneesh Hero to Chaos or adding new Hero to GK were discussed but both of them were rejected in the end for balance issues despite being envisioned in detail. So all in all, if those two didn't make it into the game because of balance wise worries from the community, it is safe to say that adding a new faction will be nigh impossible in near future.