Summarizing 3.5 years of I.FAST activities and taking into account the worldwide state of the art in various domains of accelerator science, we present an R&D plan and step-wise approach towards ultimate hadron and lepton beams. Aside more mainstream topics, we review advances in photon science, nuclear science, neutron science, high-field magnet technology, and medical applications; sketch a strategy for intense positron sources; discuss past lessons and possible directions for crystalline beams; review the strategy and requirements for “beam chaser” colliders and EDM rings; discuss the roadmap for accelerator AI, and for accelerator-based quantum computing; and we survey the actual situation and future roadmap for green accelerators and for safe CO2-free energy production. Among the highlights are new radiation schemes, novel neutron sources, and nano-accelerators.