I worked backwards to get each step.
-----The in initial thought was to be able to build super-tough structures out of diamond. Yeah I know, how flipping original of me, but since Factorio recipes are based on real world manufacturing techniques, (at least I think they are), I figured there might be an industrially feasible way to go about this based on the fact that we do currently industrially manufacture diamonds.
-----So, what do we need to be able to 'machine' diamond as a building material, or at the very least liquify it and pour it into a mold? Insane amounts of heat and pressure. Which is also what we need to make diamond if none can be found.
Diamond = Mined OR Coal + Heat & Pressure.
-----Now we need a way to produce insane amounts of heat & pressure AND contain said heat/pressure. Current diamond manufacturing methods produce small diamonds, suitable for coatings, and cutting edges. Too small to be viable as a mass produced building material. So how would we, (individual person's within a futuristic, space-age, industrial building game, AKA Factorio), go about this? Forge a star. And here's where current concrete physics are left a short distance away.
Constant Heat & Pressure = Star.
-----We know that our star, AKA the sun, is made of roughly 71% Hydrogen, 27% Helium, and trace amounts of other elements. We also know that our Sun, other stars as well, are in a state of constant Nuclear Fusion. The sun quite literally burns Hydrogen. Which brings us back to our original problem of heat & pressure.
Star = Burning Hydrogen.
-----To make a star we need a way to start Nuclear Fusion of Hydrogen and contain the subsequent release of energy. Well we can get an easy enough supply of Hydrogen by running an electric current through water in a chemical plant, (easy enough recipe), but how are we going to create a temperature high enough to ignite it? Build a Particle Accelerator.
Burning Hydrogen = Hydrogen Supply + Hyper-Velocity Kinetic Impact.
-----And now we're back on solid ground in relation to 'mechanics'. Particle acceleration has been around as early as the 1950s, possibly earlier. Seeing as how this is for an End-Game process, by then a player should have access to a surplus of resources with which to build a particle accelerator.
Hyper-Velocity Kinetic Impact = Particle Accelerator / Hadron Collider.
-----Now that we have a way to ignite a supply of Hydrogen, we need a way to contain the fledgling star lest we blow ourselves and possibly the planet we're on to bits. How do we do this? Gravity Chamber. And here's where we leave current science behind completely and have fun with science fiction.
Containment = Gravity Manipulation + Extreme Heat Resistance.
-----My thoughts on this are that it would be made out of some dense material like Lead or something, contains an unholy amount of magnets (Particle Accelerator needs them too), has Liquid Nitrogen Cooling, and is coated in a heat resistance substance, (since we're going through all this trouble to make & machine diamonds, we can't use those just yet).
Extreme Heat Resistance = Liquid Nitrogen Cooling + Graphene Coating.
-----First lets get our hands on some Nitrogen. Another simply recipe done within a Chemical plant. Hook up one or more air compressors, (converted water pumps), to a Chemical Plant, and add Electricity. Side effect of producing more Oxygen.
-----Now you're probably wondering what the heck Graphene is, even if you aren't, its a single atom thick sheet of Carbon Atoms, just under diamond in heat resistance.
Liquid Nitrogen = Liquified Air.
-----How does one go about making Graphene? Another Chemical Plant Recipe. We'll need water, detergent, and graphite. Water is easy. Graphite can be obtained by refining Coal. As for Detergent, we have yet another chemical plant recipe.
Detergent = 25 Hydrogen + 12 Carbon + 4 Oxygen + 1 Sulfur AKA 6 Petroleum(Propane) + 1 Sulfuric Acid.
-----Finally we can stop with the chemical production chain. However the above recipe leaves 1 Oxygen Molecule running free so double the recipe.
----The GC could only be placed down after and directly adjacent to the PA. It would also require a huge amount of Hydrogen to keep the star burning. And a huge amount of electricity to keep the star contained.
----The PA would only fire so long as the GC has a supply of coal to turn into diamonds or a supply of diamonds to turn into diamond parts. Should the GC not have enough coal or diamonds to complete a recipe, it would cease to supply the star with Hydrogen. After which the star would extinguish.
-----The PA would have a significant spin up time and cool down time, so starting and stopping the diamond forging/machining processes quickly would not be possible.
-----As a result of Hydrogen Fusion, the GC would also output Helium as a byproduct.
-----A Particle Accelerator in conjunction with a Gravity Chamber could be called a Solar Foundry.
-----Now with this facility running, diamond could be forged into parts just like iron & steel to construct some insanely durable products. As for all the intermediate products required to get to this point, Detergent could also be used in Chemical Plants to treat polluted water, Graphene could be used to guard things against heat higher than Ceramic could withstand but also be much easier to machine than Diamond, and Liquid Nitrogen could be used to cool other things as well as make some sort of cold based weaponry (Cryo-Capsule?).
TL:DR - Build a Particle Accelerator and a Gravity Chamber, fuel it with Hydrogen & Diamonds, and make some really tough stuff.
P.S. I spent a couple of hours googling as much as I could about the ideas herein to see just how much of this would actually be possible. Right now I'm only asking if you think this would be possible, lore wise / technology wise within Factorio. Thank you.