Ok here's a skill for you.

Coin Counter -> Throws 1 million gil all around the battlefield. Anything hit takes 50% of its current life in damage.

Gil sink and it's fun.

Or we could have a mini-gami where you throw 100,000 gil coins at a target. If you win, you lose 100,000 gil. If you lose, you lose 100,000 gil.

Gil sinks, yeah?

Also interest is dumb. Gil is easy to come by if you actually want it. There's already too much gil in the system.