There is already precedent for Valak having survived the events of the timeline of The Nun. The demon’s first appearance in nun form occurred in 2016’s The Conjuring 2, setting up the new branch of the expanded universe. That movie took place in 1975, considerably later than the events of either The Nun or its sequel, while Annabelle: Creation even teased of Valak’s existence prior to the former movie via Sister Charlotte’s photograph from sometime in the late ’40s or early ’50s.
Valak was seemingly defeated at the end of The Conjuring 2, when Lorraine uses its true name to banish it back to Hell. That being said, the demon also seemed to have been vanquished at the end of The Nun with the use of the blood of Christ sealing it away. With Valak confirmed to have survived by latching a part of itself on to Frenchie, it seems possible The Nun II could end with a similar possession situation ensuring its future survival.