Promotional materials for Aether Revolt Game Day have popped up on the internet, featuring a couple spoiled cards. A Reddit thread for the discussion thereof can be found here. The biggest reveal from this poster is Yahenni's Expertise, which reads as follows:
2BB Sorcery (Rare)
All creatures get -3/-3 until end of turn.
You may cast a card with converted mana cost 3 or less from your hand without paying its mana cost.
It's unclear if this will be the only card with this type of effect or if there will be other spells that enable you to cast other spells, though this one has definitely caught plenty of attention. People are talking about using this to cast suspend spells and split cards in Modern. A four mana soft sweeper doesn't excite me for Modern play, but speculation is often based on hype rather than results.
If there are other cards with this ability to cast other spells, we could end up seeing something that is great in Modern, though one way or another people are talking about Boom // Bust and Ancestral Vision right now. Beck // Call and Restore Balance are also getting some attention. This looks like a great opportunity to pick up some copies of these cards to flip back into the hype and/or to watch out for a more playable card with this mechanic.