Can hidden cards like Zhonyas be played normally (not hidden) in base, and what happens to hidden cards when all units leave their battlefield?

Ruling: Hidden cards can be played normally without hiding them, which avoids all hiding restrictions. A hidden card can only affect the battlefield where it was hidden.

Nuances:

• Playing a card normally (not using the hide mechanic) means it has no hiding restrictions
• Hidden cards are tied to the specific battlefield where they were hidden