I don't understand your proposal. Maybe it's because English isn't the first language for either of us...
Here's the 3 alternatives that I think you could have meant:
(1) If I have Mirror and opponent plays Rain of Fire, every creature gets hit (like un-upped Pandemonium).
-OR-
(2) If I have Mirror and opponent plays Rain of Fire, my creatures would NOT get hit (as if they were all quinted/burrowed).
-OR-
(3) Something else.
If you're proposing (1), then it would be pretty balanced buff in my opinion.
If you're proposing (2), then I'd be against it, as it would be too powerful buff in my opinion.
If (3),