A reward challenge would be a good addition to the survivor game. It will be played the same as the immunity, the highest scoring team will be given an award that will automatically be activated at day change.
Examples of Rewards:
Flint - 12 hours of fully stoked fire
Fishing Gear - double rewards when hunting for 12 hours
Feast - +10 food and +10 water for everyone on the team
The challenge will be a luck challenge every time so it doesn't just let strong tribes dominate the OT even more.