How to make a custom map

Hello! I have a custom map (literally custom, not based on real-world) and I want to draw roads & buildings on it like on OSM.

I've tried messaging the hosters of OpenGeoFiction map but couldn't get a reply back. My end goal is to make a professional-grade map of GTA:SA and host it for others to VIEW. (no editing)

I've seen a few LeafletJS plugins that lets you draw polygons and whatnot but that's not what I'm after. I want to draw buildings, roads, set building levels, road surface materials, make bridges etc.

I extremely enjoy contributing to OSM and I don't know anywhere else to ask this question. Please help me out!