The system already feels like a mech-flavoured class creator. I’d push that idea by adding a second layer of small sockets for role-defining bits: extra shoulder and knee armor, backpacks/boosters, antennas, plus dedicated weapon mounts on upper arms/forearms. On top of that, 2-3 chassis frames: a light, tall-legged scout frame, a standard all-rounder, and a heavy base that could even work without legs - more like a mobile turret. On that heavy base the torso could turn into a cockpit block with cannons instead of arms, for a pure frontline or artillery role. I’d also treat color schemes as their own module: a few preset palettes and some example builds like “fast”, “heavy”, “support” that players can tweak, a bit like picking a class in an RPG but translated into your cool robot lineup.
I’d keep it simple like this: one “hard” socket per bone for all variants, sharing the same pivot and a small box collider. The add-ons using that socket would be grouped into just 2-3 size classes (small/medium/large) instead of perfectly matching every single part - a bit of mesh intersection is fine as long as nothing pops at gameplay camera distance. For colors I’d keep all sockets and brackets on a single neutral material (steel / grey plastic) and only let armor, covers and the main module shapes change colors. On top of that you can have a few fixed palette presets as a base, plus maybe a subtle tint control for one accent color so users can nudge the look towards their project without breaking your original schemes.