Why not go for something with a bit more internal synergy? Our last surviving synergetic decision is to bank DA for raid buffs, so why not make it worth banking DA for Delirium?
A simple, but somewhat dull, version of that would be to make each EoS/FoS in duration increase the damage dealt by future EoS/FoS casts over the duration.
A cooler, but perhaps more punishing, version would be to (at longer duration) have each EoS/FoS double the damage multiplier and drain rate of Darkside (perhaps halving the additional duration granted, too?). (Or, at least have the bonus damage multiplier apply to Blood- and MP-spending skills.) Or... have it double down on but then burn Darkside pure cast. W/e.
