The Douglas Fir Group (2016) sought to articulate a transdisciplinary agenda for SLA but said little about multilingualism specifically. Moreover, many multilinguals are under siege in a worrisome world where threats to human difference have risen to the mainstream in the aftermath of Brexit and the 2016 U.S. presidential election. I argue that considering multilingualism as the central object of inquiry and embracing social justice as an explicit disciplinary goal are two moves necessary to provide sustainable support for the kind of transdisciplinary SLA that the Douglas Fir Group (2016) envisioned. I examine some missing pieces of the puzzle of transdisciplinary transformation that may make it possible for SLA researchers, and particularly those who investigate linguistic-cognitive dimensions of language learning, to contribute knowledge about the human capacity for language while supporting equitable multilingualism for all.