We all know Tsunade is Hashirama's granddaughter but to have a grandchild you need to first have a child. My question is who is he or she and why have they been hidden from us? We know the leaf village has no problem hiding ones lineage from them, because Naruto was never told who his parents were he found out himself. Who Hashirama's kid is will certainly come in to play trust me. Maybe he's Tobi after being twisted by Madara it's the ultimate F U to Hashirama, to corrupt his child.