Other option could be a new setting based in "shards" or fragments of the First World, a patchwork demiplane within the elemental limbo, adding a lot of thigs from "Councyl of Wyrms" and some pieces of Mystara Hollow World spin-off. This could add enough space and "hunt territory" for all dragon species and even future new kaiju monsters.
Time travel is canonilally possible in D&D but they are extremely rare because the cosmology could be too easily altered. If WotC wants to recover the chronomancers then they will need a lot of time and work to understand how the future metaplot could be affected.