If she sacrificed herself. I still haven't given up hope that she will someway, somehow, for whatever reason pull a granny Chiyo.
But truthfully, what would make her useful if for Kishi to stop making her useless. She already has the skills to be useful. She's already being useful what with the whole keeping ppl alive thing and all (that's kind of important in battle).
But within the context of an actual fight which is what ppl seem to mean by being "useful", I'd say actually engaging in the actual combat instead of just one shotting an opponent then standing like a deer in headlights all shocked that the enemy actually counter attacks.
Yes, here's how she can be useful: actually be ready for a counter attack and not have to be saved when the enemy actually punches back.
She can also save Naruto and Sasuke in a fight for once. How about covering their backs for once. What, an attack from behind? Keep your eyes in front of you Naruto/Sasuke. I've got your back.
Or better yet. Isn't she like some sort of genius with all this book knowledge? How about coming up with a god damn plan of attack for once? If she's the brains, why the hell isn't she anayzing the enemy's movements (especially with Tsunade's evasion training) and conveying what she analyzed to Naruto and Sasuke so they can counter attack appropriately?!
****ing Kishimoto. He tells us all these things about her, but he refuses to show them. Why?!
Just like in the Sasori fight, he kept ramming "Oh what an impressive girl" down our throat instead of SHOWING US and letting us cone to that conclusion. Such cheap writing. Oh well.