A single man splashable powerhouse that needs to be dealt with immediately.
other goblin creatures you control attack each turn if able.
at the beginning of combat on your turn, put a 1/1 red goblin creature token with haste onto the battlefield.
whenever goblin rabblemaster attacks, it gets +1/+0 until end of turn for each other attacking goblin.
There are very few aggro decks that wouldn't run this. It's not often you see a card that's so defining in Standard, but the Rabbler's going to continue to rise in popularity (and price). Also. combos with Mardu Ascendancy, of course.