Another suggestion would be to add something like Der Stuka - it's just a steep body slide, and it could even be paired with a shorter trap door slide, which would help alleviate the wait for Kala & Tai Nui - perhaps their could be a smaller mountain (about 60 feet) that serves as their platform.
The "action river" suggestion above may cover this, but a lot of water parks outside the US, especially in Latin America, have a slide that resembles a river - very gentle change in elevation, but with the occasional sharp turn and rapids. They are not loops, meaning there is a beginning and end to the ride, but it would pretty easy to integrate into Volcano Bay's theme.