Intercosmic Shifting

As you cast this spell, you may exile it face down.
For as long as this spell remains exiled, the first
spell each player casts is exiled face down as it
resolves
{4}{u}{b}{r}: You may cast all spells exiled with
Intercosmic Shifting without paying their mana
costs. Return them to their owner's respective
hands afterwards
"What should we add next?"