When Warhammer and open-source collide, I'm always fascinated to see which aspect the discussion assumes deserving of attention: the miniatures, or the
rules. The minis were historically protected by the rarity of injection moulding hobbyists, but in the era of 3d printing all bets are off. It's made easier by the casual Warhammer scene having long ago grown accustomed to "counts-as" models. Assuming it's around the same size and profile, you can use that WWII tabletop tank and call it a Leman Russ, so a Leman Russ with sliiiiiightly off dimensions or print quality is a shoe-in.
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.
[0] https://en.wikipedia.org/wiki/Vassal_Engine