For the next 40 seconds, your basic attacks hit any enemy in range.
or
For the next 40 seconds, you get +10 to your tactical DCs, +10% to dodge and dodgecap, +10% to damage, immunity to stun/paralyze/hold/etc.
or
For the next 60 seconds, your tactical abilities get +100 MP
or
The next 100 attacks you make get +100 MP
Personally I think any of them are more interesting than just +damage. If you keep it at stacks, work in +tactical DC in the stacks.