The express intent of this scenario is to maintain the House of Habsburg in the male line while maintaining the reforms/legacy/influence of Maria Theresia in some fashion, without the existence of an illegitimate/natural line (too easy)
So, if say Karl VI has a son later in life, say 1730, but the boy is sickly, so he continues to campaign for the recognition of the pragmatic sanction just in case.
Karl VI dies same time in 1740, the boy is around 10. Francis is elected Holy Roman Emperor as the boy is considered too young but the court of Austria fearing the influence of Elisabeth Christine/House of Welf make Maria Theresia regent for her younger brother instead while leaving the boy in Elisabeth Christine's care.
Additionally the young child's mental capacities are in doubt, he weds (not picky as to whom) when he achieves his majority in 1745 but effective control of the govt. remains in Maria Theresia's hands. He is fortunate to have twin son's in say 1747 but both him and his bride pass away in 1750 leaving the children in Maria Theresia's care and so her regency continues until the eldest twin (both live) achieves his majority in 1760/61. (She remains a trusted advisor and holds strong influence over the new Archduke, who eventually becomes HRE when Francis I dies in 1765)
How plausible is such a scenario? Reactions from other powers? How would Maria Theresia be viewed in such a scenario? power hungry or great caretaker? (very convoluted means but thought I'd put if out there) With regards to external powers I think the imperial election post Karl VI will be very interesting.