Tobirama said he wanted to travel the world and take a look around. He also said that when the village needed him, he would naturally return.
In truth, he found it hard to let go of Konoha. He surely still wanted to take control of Konoha again, but opposing Shinichi now was hardly a wise move. It would only hurt Konoha.
He had already touched the threshold of immortality. There would always be another era that belonged to him in the future, so he might as well try living differently for now.
Tobirama recalled something his elder brother had once told him.
Returning to the Living World had made him too impetuous and self-centered. He needed time to settle himself. Traveling through the Shinobi World was a good choice, and he could also search for a way to repair his soul.
In the future, he planned to summon his elder brother with the Impure World Reincarnation once more, then revive him.
That was what he wanted, but whether Shinichi would give him that chance was another matter. Once something belonged to Shinichi, how could he possibly hand it over to someone else?
After Tobirama left, Shinichi took another stroll through the laboratory.
Everything here was far too specialized. Shinichi could not make heads or tails of it, nor did he have the time to keep tinkering with such things.
He went to inspect the test subjects. After giving it some thought, he summoned the Anbu and had them place the Cursed Tongue Eradication Seal on these people before sending them off to live in some random small town.
Soul Reincarnation, soul division...
In the original story, Orochimaru had become incredibly weak because of Living Corpse Reincarnation. It was probably for the same reason, only the method of casting it was