We all know gear 2nd it's a pretty know-well power up in the anime, and It would be cool if this will be added in the game.
My idea is a about a power up that increases the damage of all the techniques from the normal rubber attacks, making them more faster and stronger. Of course this will sound broken, so we can limit it with a "Residual damage" Of using the power up like the anime because:
"Gear 2 dramatically shortens Luffy's life force, increasing the chance of having a much shorter lifespan due to starving his body." And we can translate that about making the player energy bar go down constantly.