The rules, though, are a different matter. Warhammer 40k (the sci-fi variant) is a confusing blur of simultaneous, competing standards, controlled by an edition rulebook (which handles general gameplay rules and discussing physical aspects of the hobby like collecting and painting) and Codex books for each army, which aren't necessarily updated when a new edition comes out. The interactions of the army rules in each Codex and each edition's ruleset is chaotic at best, and often results in insane-looking letter-of-the-law strategies that exploit that interaction (early Tau firefish, anyone?), until it's fixed by a new Codex or edition.
The problem is, IMO, theorycrafting. The nerds that wind up playing your game have a lot more time, in aggregate, to break it than you do, and once one 'sploit has been found, disseminating it through the internet is trivial. Now everybody's doing it, and it's called "the current meta". If you don't want that, you've got to change something. You can add ever more and more complex pieces to the system, like they did with D&D3.5, or you can keep it moving forward and correct newly discovered imbalances live, as they do with WH40k (or, if you're a conspiracy theorist, correct some and intentionally introduce others).
I stopped trying to climb the endless mountain of 40k a long, long time ago. While I idly dream of a drop-in replacement ruleset that does away with the push-and-pull of edition-and-Codex rules-leapfrogging, I don't think something as complex as 40k can really be distilled into a timeless, unproblematic ruleset like chess or Icehouse or Homeworlds, especially if the intent is to continue releasing new models with new rules... especially if they're Space Marines, the Mary Sues of the 40k universe and the best-selling model line by far.
Anyway, I find it interesting that the parent comment took the approach it did. Note how Warhammer-as-open-source is about the free market providing new models, rather than the reverse-engineers "pirating" the entirety of Magic for XMage or creating new rules for the Star Wars variant. Warhammer has an "alternative client" too, in the form of the Vassal engine [0] (an LGPL 2d tabletop simulation system). Unlike XMage, it doesn't automagically enforce all rules, but then, neither does IRL live-action Warhammer. One could do something similar in Tabletop Simulator, of course.
We all know and laugh at "illegal primes", but my intuitions, about how this applies to (what will eventually be, at its final stage) a physical object, are confusing me.
That said, access to low-cost, high-resolution 3D printers (specifically resin-based printers) has enabled an explosion of the open-source Warhammer community (see https://www.reddit.com/r/PrintedWarhammer/ for one example) -- and oddly, it's exactly the free market that is allowing this to flourish through services like Patreon, where artists are financially rewarded by supporters who receive a steady stream of fresh new .stl's on a regular basis.
Open-source Magic the Gathering has struggled a bit, but there are still a good number of fan projects that have done very well. In particular, open-source game clients such as XMage have been open-source competitors to MTGO, and open-source fan-project expansions such as Star Wars: The Gathering have managed to keep a high level of quality and consistency: https://www.starwarsthegathering.com/