How about letting the wizard be even more of a gambler "mad" with Retributive Strike. It can be used at one of three levels, normal, mad, and insane. Caster must declare which level is being used before the roll.
Normal - Roll 20 100 damage to all creatures in room, roll 16-19 75 damage to all creatures in room, roll 11-15 50 damage to all creatures in room, 6-10 25 damage to all creatures in room, 2-5 no effect, 1 wizard takes 25 damage
Mad - Roll 20 150 damage to all creatures in room, roll 16-19 100 damage to all creatures in room, roll 11-15 75 damage to all creatures in room, 6-10 50 damage to all creatures in room, 2-5 wizard takes 25 damage, 1 wizard takes 50 damage
Insane - Roll 20 200 damage to all creatures in room, roll 16-19 150 damage to all creatures in room, roll 11-15 100 damage to all creatures in room, 6-10 wizard takes 25 damage, 2-5 wizard takes 50 damage, 1 wizard takes 100 damage, all members of party other than wizard take 25 damage.
The exact damage numbers could be adjusted. Just proposing this as an fun concept and for the wizard who wants to dial it to "11" and potentially earn the wrath of the party.