Neodymium-based permanent magnets (Nd₂Fe₁₄B) are widely applied in technologies such as electric vehicles and wind turbines due to their high magnetic strength and resistance to demagnetization. Despite these advantages, their performance is limited by a low operating temperature (<585 K) and poor mechanical properties. This study explores the influence of rare earth substitutions, specifically Gd and Ce, on the structural, electronic, mechanical, and magnetic behaviour of Nd₂Fe₁₄B using ab initio density functional theory. Thermodynamic stability, assessed via heats of formation, indicates NdCeFe₁₄B as the most stable composition. Partial substitution with Gd enhances ductility, mechanical stability, magnetic strength, and Curie temperature, offering a pathway to improved permanent magnets. These insights support the design of advanced magnetic materials with higher operating temperatures and superior mechanical performance.