Karn stayed in the Shinobi World for over a year. Only after the reforms in the Land of Fire had stabilized and Tsunade's belly had grown larger by the day did he return to the Marvel Universe.
Although leaving when Tsunade was close to giving birth was not ideal, Karn still had plenty of matters piled up in other worlds. In the Resident Evil World, for example, as Alice's godfather, he had to check on her growth periodically, as well as the progress of the aircraft carrier's construction.
The aircraft carrier he wanted to build could practically serve as a complete space city, with an internal energy supply comparable to four hundred-millionths of a planetary core. Karn had also made some alterations, adding more alchemical runes inside to ensure that it would automatically repair itself when physically damaged.
Such an enormous project was impossible to complete in the Marvel Universe. There was not enough underlying capacity, and even joining forces with the United States government would not be enough to build it. There was only some hope in the Resident Evil World, where the Umbrella Group's immense power and resources could influence political transitions in several major powers.
If people had to choose between evading government taxes and Umbrella's products, everyone would presumably choose the former. The latter were like air, products indispensable to human survival. Even after death, people might still purchase funeral products that Umbrella had invested in.
"The haul was pretty good, too. All of Eight-Tails' chakra, plus some of Three-Tails' chakra. If I fight using the Rumble-Rumble Fruit's abilities, I can keep going for at least three days or so. Even the most basic electrical discharge should reach five hundred million volts."
Moreover, the changes Eight-Tails Chakra brought to his body were still slowly improving it.