I was thinking, that Yavanna at some point should stay a little with Legolas and Tauriel. So Yavanna/Legolas, can mainly develop their relationship through the story...but she'd still be united with the company, during the Battle of Five Armies at some point. But I see the trio*Yavanna/Legolas/Tauriel*, being good company and fighting together as a team, during the battles. Aswell as Yavanna helping the company in the battle, of course.
But I think Maybe Bofur and Saeril might suggest, she stays behind when they see her around Legolas and what not :) She might be torn between staying and going. So they'd just offer her some advice, and the general..'keep her safe', speech to Legolas.
You know, the scene where Kili/Tauriel were saying goodbye. I was thinking that'd be re-done to a Yavanna/Legolas scene, but Yavanna would be reluctant to say goodbye, so that's when Saeril/Bofur do the above. Maybe Kili/Fili might add in a few words and all :) <3