Then, in theory, the Habsburg dominions would be inherited by Josef I's eldest daughters, Archduchess Maria Josepha, as per the 1703 Mutual Pact of Succession. Though I imagine if its obvious that Karl VI won't have children Maria Josepha will have a different husband then OTL. Or, Karl VI could end up petitioning for an annulment. After all, its easier to "divorce" a childless spouse than one who's produced at least daughters.