If mass land destruction is appropriate at your table, you could do a lot worse than Carmen in the command zone
Im a sucker for a good Impact Tremors, but you can also get life from Soul Warden and her sisters, card draw from Garruk's Uprising, and mana from cards like Gala Greeters and Galadriel, Gift-Giver
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This includes the commonly played mana rocks that tap for mana as the act of using their abilities are activated abilities
You could do something as simple as adding an extra -1/-1 counter to an enemy creature, or you could really ruin someone's day by doling out poison counters at an alarming rate
Celebi Celebis Traverse Time attack could be a really solid pick for Grass box decks especially those running the powerful Mega Venusaur ex strategy