Original (nearest neighbour):
Bicubic (more or less, gradients between the pixels):
Original & HQX (good for pixel art):
There are other approaches to like vectorisation, neural networks, things with inpainting in various ways, things with the spacial/frequency domain.
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Any of thease can be pre-computed, or resolved at least to some extent on the fly as you scroll the mouse wheel and zoom in, so that could an interesting thing to do with a mod, I would also imagine that this would not conflict with others mods as it just increases the resolution of the textures, for buildings, belts, items on belts etc... It doesn't need to be excellent to give an interesting artistic effect, and it's all automatic.
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I have an idea that I've experimented with myself, with unfortunately mixed results, but I feel is a resonably solid idea, there are so many weights and variables, so many ways that it could be implemented that perform differently.
Original (sorry if this doesn't seem very pixelated on small monitors):
Enhanced:
The concept is that large regions of the image are taken, either systematically or at random, shrunk down and superimposed over smaller regions of the image that they resemble.
Most objects are less obviously recursive, but I think often are to a reasonable degree even if it is not obvious to us humans.
The performance would improve considerably if matches from one texture could be painted over others, not just itself.
This should change a temporary version of the textures, not permanently alter the factorio texture files.
