Naruto sighed as he drew the last line of the seal. Sasuke had wanted closure after everything was all over, and since they couldn't re-summon the Nidaime through the Edo Tensei because doing so carried an automatic death sentence, this was the next best thing. Fortunately, time being what it was, any actions Sasuke might take would form an entirely new timeline and therefore wouldn't negate their own timeline and their own existence. With that in mind, he'd chosen to allow it on the conditions that Sasuke not destroy Konoha or kill the Shodai or Nidaime Hokages or Madara.
The instant the seal was complete, Sasuke stepped into the array and indicated that he was ready to go. As he activated the seal, the door to the room slammed open and Sakura raced in.
"Naruto!" she yelled frantically "Don't send him yet, Sasuke's got the weaponized..."
But it was already too late, Sasuke was gone.
Konoha, many years in the past:
The festival that marked Konoha's first Spring was in full swing. Just about everyone was glad that the Winter was finally over, and in something of a mood to celebrate irrespective of clan affiliation. The main thoroughfare that had been decorated for this somewhat impromptu festival was packed with people, or as packed as the place could get considering the size of the newly formed village. For the most part, said people clustered together with members of their own clan, but there were a few mixed groups, most of which consisted of children.
Tobirama walked alongside his brother who had a nod and a happy smile for everyone he encountered. He however had none such to give, and what little good mood he had soured even further each time he saw one of the mixed groups running around. As far as he was concerned, welcoming the Uchiha into their midst was as dangerous as clutching an adder to one's breast, and would eventually have the same end result. He honestly didn't know why his brother couldn't see this despite the fact that it was staring him in the face.
While Hashirama was busy inspecting a frog that one of the little Uchiha bastards had held up for examination and complimenting the little brat on his lame-ass hunting skills, a seal that looked as if it had been created by a drunk Uzumaki appeared in an empty spot in the middle of Konoha's main street, along which the various stalls that sold food or trinkets or offered other diversions such as fortune-telling had been arranged. As a number of people turned to see what was happening, a Uchiha who was carrying a glass jar appeared in the middle of the seal, and scanned the crowed as if he were searching for something - or likely someone - until he locked eyes with him.
The instant their eyes had met, the most venomous look came across the younger man's face. Glaring, he returned the challenge with every bit as much venom as he received.
"Well Nidaime-sama, you got your wish, I'm the last of my clan." the Uchiha said coldly as soon as he was confident that he'd managed to get his extreme hatred for the Senju across. "And, I have just one thing I want to say to you."
"And, what is that?" Tobirama asked as he armed himself and prepared to close the distance between the two of them before the man could harm any of his clan.
"Payback's a bitch!" the Uchiha said with a feral smile as he dashed the jar against the ground in front of the seal an instant before he and it both vanished.
An instant later, the jar's contents were released into the air, and a pink miasma settled over the village. Moments after that, all hell broke loose.
Nine months later:
Tobirama scowled down at the small silver-haired bundle that had been set in his arms. Five blood tests which had been administered by the Uzumaki who had been employing such seals for generations had confirmed that this half-Uchiha thing that had been conceived during the indiscriminate mixing that had taken place during that festival fiasco was his offspring.
As he turned to hand the small creature back to its mother and walk out of its life forever, an Iryo-nin walked up to him with another small bundle.
That made three so far...
"Mito's never going to forgive me..." his brother groaned from down the hall where he was gently cradling an abomination of his own. Sitting next to him, a certain spiky-haired bastard watched in anticipation as a volunteer who had come to help the swamped Iryo-nin approached with a pair of bundles whose small heads were covered in a mass of spikes.
"Twins!" that bastard who'd dared pick up his friendship with his brother as if there hadn't been decades of bad blood between them exclaimed, his eyes lighting up with an unholy glee.
Well, at least someone was happy about all this...
In the future with Naruto and Sakura:
"...Sex pollen." Sakura finished as a smirking Sasuke re-appeared.