To originally cast the spell hex, you must target "a creature you can see within range". The spell says that if the target drops to 0 hit points before the spell ends, the caster can use a bonus action on a subsequent turn to move the curse to a new creature. However, it does not say that the new creature being cursed has to be seen or within range.
If the initial target of hex is reduced to 0 HP, can the curse be moved to an unseen target or one out of range?
Rules as Written, it seems possible to do so, but that seems a bit overpowered.