I know I'm late to the party for Ravnica spoilers, but now's a good time as any to hop back in for more first impressions & deck ideas:
This card is bonkers. I don't usually like straight up green/white, but when I do, it's usually with cards that fit into multiple fun casual archetypes with the ability to go off and do crazy things. And Trostani, Selesnya's Voice does all of these things. It's a 4 mana 2/5 creature with a reusable populate ability. And I don't know about you, but I think populate is easily the most busted of the new Return to Ravnica mechanics.
I've always been a big fan of life gain decks, but the trick with Trostani is to fit her (?) into a life gain deck that already uses a lot of token creatures. The first card that comes to mind is Ajani Goldmane.
If you ever get to ultimate Ajani Goldmane, not only will you create an avatar token and double your life total, but you'll be able to create an avatar token and double your life total every turn.
Sounds like a plan.