We need a conceptually different idea of cities which paradigmatically shifts their forms and structures toward more liveable future environments, both for us and our planet. The Isobenefit Urbanism is conceived within a medium-long term perspective mixing a macro top-down planning with a micro bottom-up spontaneous evolution in an attempt to moderate the human forces which typically induce agglomeration benefits and costs, maintaining the former while limiting the latter. It indicates a simple efficient urban genetic code to generate cities that are walkable, carless, low carbon, adaptive, connected, compact, multifunctional settlements throughout nature, with unplanned forms and unlimitedly extendible, in which one can feel both urbanity and nature. They are flexible cities and dynamically changeable. By holding constant the number of inhabitants of a usual city or megacity, its counterpart Isobenefit city would enjoy the same economies of scale benefits but without their costs. The Isobenefit Urbanism model might offer a potential solution to current issues of wild cementification, urban heat island effects, destruction of natural land and biodiversity, carbon emissions, congestions and related air pollution, as well as provide an urban model to host the enormous new urban inhabitants our world must accommodate in the next few decades. Its ultimate ambition is to enjoy the economies of agglomeration without incurring to the diseconomies of agglomeration, manifested by sublinear and superlinear outputs, typically infrastructural the former and socioeconomic the latter.