I like the idea of Fling for RDW. Trying to think if it's worth cutting anything already in there though. Probably want another couple Brimstone Volley as that seems like a finisher for this deck when your creatures start dying.
I know it's a controversial card, but how would a few Vexing Devils go?
A great combo I would include in a RDW is Kruin Striker + Thatcher Revolt . The kruin strikers toughness will always stay at 1 with that combo, so even if it gets chumped you have trample damage that is gonna go through then you have another way to activate the morbid on Brimstone Volley .