Just for an example for the riposte idea.
Let's assume a low-power character.
Four shifts of power in a shield. Let's say the control roll was a 5.
What would the various attempts to riposte look like?
Initiative:
Attacker
Wizard
Round 1: Attacker hits wizard, this is blocked by the shield, Wizard can't convert this round.
Round 2: Attack misses wizard (without the wizard needing to use the shield), wizard converts the power into an attack (flavoring it as reflecting the attack energy, perhaps).
or...
Initiative:
Wizard
Attack
Round 1: Wizard puts up shield, attacker would hit wizard but the shield blocks it.
Round 2: Wizard converts shield into an attack, perhaps flavoring it as reflecting the attack energy.
Note: In this instance it doesn't matter if the attacker forces the shield to be used or not, since the wizard always acts before him in a given exchange.
In both cases the wizard need some sort of justification based on the flavor of how the shield works for how he is converting it into an attack.
This is using initiative by RAW.