I think it's possible that Sakura will have a slug sage mode.
But I don't think it could happen now.
Slug Sage mode just seems too.........copy righted from the frog Sage mode to even happen in my opinion. The snake/dragon Sage mode that Kabuto has was enough to happen as it is so hopefully no more animal based Sage modes. X)
If Sakura were to achieve Sage mode by money is on Hashiramas version of it. Not in having any blood or spiritual connection to him but in considering his Sage mode may have a connection to his wife Mito Uzumaki. Who also seems to have a Yin Seal on her Forehead.(though there is no evidence to back this up it's just a theory I strongly believe in)
Plus if the Edo Hokages are still around Kishi has a plan for them in the final battle. My best guess a cooperation with each hokage to each member of Team 7
-Naruto/Minato (fourth hokage) rasengan users
-Sasuke/tobirama (second hokage) Speed and teleportation users
-Kakashi/Sarutobi (third hokage) 5 elemental Jutsu users
-Sakura/Hashirama (first hokage) Regeneration and (hopefully!!) Sage mode users
They would all strengthen each other's attributes very well and would make a great development into the final battle for all the fans to enjoy.