First of all this is just a theory, so please don't attack me :(
I believe she has Hashirama's wood style because:
1-Her name means "Cherry Tree" and trees are made of Wood.
2-RtN Kishi's interview he speaks about Sakura's parents and he mentions about them being outside merchants. No more background for the Haruno clan.
3-Yamato told Sakura she couldn't help Naruto because he was the only one who could control the Wood Style. Maybe because they were so focused that he was unique that didn't mind looking for another one?
Sakura Databook Element:
https://31.media.tum...JdqX1qbs7t8.jpg
Translation:
http://img1.wikia.no...7d/Elements.jpg
And as you can see she has both Water and Earth elements.
4-Parallels. On the fight with Madara, Hashirama received help from Mito as a Jinchuuriki, right? So it was:
-Uchiha as the villain;
-Uzumaki as the Jinchuuriki;
-Senju as the Bijuu controler.
We already have:
-Uchiha Obito/Sasuke as the villain;
-Uzumaki Naruto as the Jinchuuriki;
-Senju...
So who is the Senju?
5- Tsunade.
Tsunade is both Senju (grandfather's Senju Hashirama side) and Uzumaki (grandmother's Uzumaki Mito side)
Wood style:
http://naruto.wikia....ki/Wood_Release
Hyakugou:
http://naruto.wikia...._a_Hundred_Seal
Notice the Hyakugou it's only a Chakra storage to perform techniques and its only used for healing with Tsunade.
And according to this article the Uzumaki helped the Senju with Fūinjutsu:
Which is a jutsu to seal everything into anything. That includes Chakra as well.
6-Surpassing the master.
Tsunade once mentions that she didn't innerate her grandfather's regenerative cells.
If Sakura is the Senju with Hashirama's Wood Style, that couldn't make her the one with the regenerative cells? That way she will be the best medical nin and surpass Tsunade.
7-Haruno Clan: (just to add because someone mentioned)
Hashirama's forehead:
http://img.mangastre...uto/621/013.png
Haruno Sakura:
http://25.media.tumb...v8v4go1_500.png
Edited by Kate Anesuna, 15 December 2015 - 02:30 AM.