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Two-dimensional (2D) ferromagnets with high Curie temperature TC are desirable for spintronics applications. However, they are rarely obtained in experiments mainly due to the challenge of synthesizing high-quality 2D crystals, and their TC values are below room temperature. Using first-principles calculations, we design a family of stable 2D FenGeTe2 (4 ≤ n ≤ 7) ultrathin films with coexisting itinerant and localized magnetism. Among them, 2D Fe3GeTe2 and Fe4GeTe2 are ferromagnetic metals with TC = 138 and 68 K; 2D Fe5GeTe2, Fe6GeTe2, and Fe7GeTe2 are Néel's P-, R-, and R-type ferrimagnetic metals with TC = 320, 450, and 570 K. A thickness-induced magnetic phase transition originates from competition between itinerant and localized states, and also correlates with Fe3+ and Fe2+ content. A valence/orbital-dependent magnetic exchange model is proposed for these effects. Our results reveal a universal mechanism for magnetic coupling in complex magnetic systems. 2D magnetism offers the prospect of ordered and controllable magnetic states at reduced dimensions and there is a desire to obtain materials which can realise these properties at room temperature. Here, the authors use first-principles calculations to demonstrate that a family of 2D FenGeTe-based ultrathin films with varying Fe content can exhibit a range of magnetic properties at elevated temperatures. [ABSTRACT FROM AUTHOR]