Omega Force's sandbox experience helped inspire its partnership on Pokémon Pokopia.
Game Freak partnered with Omega Force because Omega Force had experience developing sandbox-style games, particularly Dragon Quest Builders 2. That background was relevant to Pokopia's focus on gathering materials, changing terrain, constructing buildings, and creating player-designed habitats.
Omega Force is part of Koei Tecmo and is widely associated with action games, but its work is not limited to one genre. For Pokopia, its sandbox knowledge complemented Game Freak's experience with Pokémon characters, creatures, and world-building.
Game Freak remained one of the game's co-developers, so the answer is not “Game Freak.” The question asks which partner brought the cited sandbox experience to the collaboration, making Omega Force the specific answer.