Before the next loop begins, here's a heads-up: the premise for this loop is seriously dark and takes huge liberties with canon. If you like Uni and Checker Face, please read this first before deciding whether to keep going...
First, a warning in bold: this was inspired by that bleak novel, Calamity of Tomorrow. Honestly, how did the author have the nerve to call it a lighthearted story? The truth reveal practically made me cough up blood.
Then I thought of how, in the original Katekyo Hitman Reborn!, Checker Face said he was a true Pure-Blooded Earthling, but only he and Uni remained of their kind. Before modern humans came into existence, they had created the Tri-ni-sette to protect Earth.
That raised a question for me: in essence, the Tri-ni-sette belonged to the Pure-Blooded Earthlings, so its allegiance was already a little questionable. In canon, all the other Pure-Blooded Earthlings had passed away, but judging by Checker Face, they actually had very long lifespans. What if, in one world, the Pure-Blooded Earthlings hadn't died, but had instead chosen to enter a deep sleep as Earth's environment gradually became unsuitable for them? They used the Tri-ni-sette to create an alternate space (which was also one reason they needed Human Pillars). The Human Pillars really were needed to protect Earth, but what they were actually protecting was the Pure-Blooded Earthlings' Earth, allowing it to gradually return, over time, to a state suitable for their kind... And when the Pure-Blooded Earthlings returned to Earth and discovered that modern humans had already taken it over, what would happen?
Peaceful coexistence? I doubt any of you believe that. Of course they would fight to the death, until one side had exterminated the other. And since the Pure Humans (Pure-Blooded Earthlings) were hidden in the shadows