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[Hide] (259KB, 735x374) Reverse >>116745
you will get far, faaaaaaar more enjoyment from modding an existing game than from building an engine from scratch as the latter is too much for 1 person (that's why dev engineers get paid $200k a year each to do this as a team)
if you go the 3d route, you'll have to account for: physics, textures, lighting, shaders, 3d models, animations, particles, sound systems, user interfaces, npc pathfinding, multiplayer netcode, rendering optimization, level designer and modeling tools, and general game logic at the very minimum
ultimately, you will just end up reinventing the wheel as existing engines already do all of this (what I listed was for valve's source games)
you start from scratch only if you want to completely own your product and be free to sell it however you please, but even then, there's a good chance you can make revenue from modding anyway if you choose that route