Reincarnated as a low-ranking demon in Hell for two thousand years, a man with memories of being Japanese was enjoying a slow life. However, suddenly, a war broke out between Heaven and Hell. Caught in the aftermath of the power unleashed by the gods against the Demon King, he was swept away to another dimension.
Lost in the dimensions and eventually finding himself in a different world, the pitiful low-ranking demon stumbled upon a human child.
“I have some free time, so I’ll raise you.”
Driven by nostalgia for his human past, the low-ranking demon decided to whimsically raise the human child, unaware of the truth. This child, in fact, was the legendary hero destined to save the world.
But it was about one powerful world (the MC being a son of a smallish noble) who would monopolise rifts into other worlds/planes where they would trade/conquer/explore/develop worlds, extracting resources and trading / battling in the main world for position with the other nobles.
It was years ago but I really enjoyed it and it was heavily about developing, I think he enters his first sub-world thing with a handful of people and builds it into a nation with armies and so forth.