Yeah, well that was the requirement for the Sage Mode, though we never know.
I still think that Slug Sage Mode will come to play as Kishi keeps patterns and we already saw that Snake and Frog have, it would be highly unnatural for Kishi who has done this countless times, not to follow the same pattern and develop Slug Sage Mode.
However, don't know to who is going to be assigned to. Tsunade or Sakura?
I do incline to think that it will be given to Tsunade given the fact that the body is not a problem for her, inheriting the Sage body via Hashirama. And we're being proved right now that she is the most resistant one in the whole Narutoverse. We haven't seen anyone being cut in half and survive. Excepting Hidan who is part of his jutsu. Tsunade doesn't even have a jutsu activated.
Tsunade using Slug Sage mode now, actually comes more in handy than anything else. Why? Because at the beginning, all they had to do is seal him or keep him at bay. None knew about his abilities or stuff like that, so using the Sage mode right from the beginning it would've been not such a smart move. And we know that Tsunade is anything but stupid.
Actually when he pulled Perfect Susano'o there was this little small panel where it showed Tsunade preparing to do a hand-seal, however, at that point, supposedly Madara would've been gone as EDO was cancelled.
Once again, why use Slug Sage mode if not needed?
No one knew that Madara was about to break free from the EDO.
Now, the whole Tsunade in half and the Kages beaten. That was a necessary move to show how superior Madara is.
But by keeping her alive, Kishi is sure foreshadowing something.
Plus, not to mention Byakugou's abilities that Kishi showed us and the panel where Dan says not to join her.
Oh, and her statement: I'll never die in battle.
It will be plain stupid to kill her in battle, actually.
And the only time we saw Tsunade near death was in Pein arc, but because chakra exhaustion, not a battle.
So I am sure she will not die and come-up with something.
BUT that is a 50% chances.
Why?
Because Kishi said Sakura needs to grow. Every character from the newer generation that needed to grow had their masters killed so here it is the trick. Tsunade lives or doesn't?
My guess would be that Sakura's growth will be different than the other's. Meaning, once and for all, she'll get past the only thing that hold her temper down, as Kishi said in a manga page, her heart.
Meaning, probably Sasuke.
The only times we have seen Sakura being hold back was when Sasuke came into play so her growth might strongly be connected to that.
After reading Kishi's interview and the manga and making connections, these are the possibilities I came up with.
Hopefully we will get to see something interesting.