Your spatial data needs a spatial reference. Then you can use Leaflet/openlayers. The spatial reference doesn't need to be real, especially if you don't care where it gets placed 'in the real world'.
No spatial reference = not spatial data.
If you want to persist with your data NOT having a spatial reference, then you don't have spatial data - you just have pixel values (or similar). I mention this because this may help you frame your question or look for a more appropriate solution, like a graphics/photoshop type solution an dyou may need to move away from having multiline datatypes. .
But giving your data a spatial reference and then using leaflet/similar, is actually quite easy!
I would personally just play around with a map projection that roughly fits somewhere you like in the real world - It doesn't matter where! Pick New York/England/Moscow, who cares. Just somewhere. Then find a map projection that fits that area (you can look for them here https://epsg.io/
You just need a reference EPSG number.
have a look at ICRS in the Leaflet documentation. https://leafletjs.com/reference-0.7.7.html
"Defines coordinate reference systems for projecting geographical points into pixel (screen) coordinates and back (and to coordinates in other units for WMS services). See Spatial reference system."
"L.CRS.Simple A simple CRS that maps longitude and latitude into x and y directly. May be used for maps of flat surfaces (e.g. game maps). Note that the y axis should still be inverted (going from bottom to top)."
Game maps are very commonly just a set of pixel values similar to your data, which people project into leaflet.