This spell has an interesting push-pull to it since you often don't want to use it as both a regrowth and a
Return to Nature unless the game goes long and you can grind out value. Here's a couple cards that are comparable to help narrow down a cost:
So the forecast is a sorcery speed, but uncounterable
Return to Nature with buyback
? That seems really oppressive to artifact and enchantment decks, if you support those.
Agree with Onderzeeboot, so depending on your environment, looking at 4-5 mana for the forecast ability. Since the forecast is so expensive, you can probably make the casting cost just
to use the regrowth effect; if we go with 5 to forecast, it would come to 6 mana for 1 card, 11 mana for 2, etc. though over the course of multiple turns.
To bring it all back around, my suggestion with the design as proposed would be:
Grand Remembrance -
Sorcery
Return all cards exiled by cards named Grand Remembrance to their owner's hands.
Forecast —
, reveal Grand Remembrance from your hand: Exile target artifact, enchantment, or card in a graveyard.