Why not rock the boat and produce the game's first TAoE DoT?

Pick a target, they explode every 3 seconds taking damage and dealing damage to all targets around them. Only one instance can be active at any time.

There you bypass the need to try and multi-DoT as well as get around any issues faced in ST situations. Whilst also, providing something new and unique instead of Shadow Flare 2.0.

Could then give it Mana per target hit so as to amplify Mana gain exponentially in AoE situations to provide more Moulinet combos. If needs be, put an upper cap on amount of targets will generate Mana to limit how crazy it can get on MASSIVE pulls or something.

As far as a Verfinisher goes... I think an AoE Verfinisher seems awkward. Given that AoE Mana generation is centred on filling up both types of Mana, which would really only lead into having a single Verfinisher. To which it would be kind of awkward trying to pick whether it should be a Black or White spell. Given RDM's ST rotation being appropriately balanced in terms of Black vs White spells.