To expand a bit on my comments here on how it's difficult to write new rules for GURPS:
It's not hard to add entirely novel rules to GURPS. The problem, in my own writing and gaming, is finding a need to add entirely novel rules. Mechanically, it's not necessarily hard to write new rules. GURPS is heavy on "find the appropriate trait, modify for circumstances, and roll that number or lower on 3d6." And there's always room for special cases. For example, Tactical Shooting and Technical Grappling add a wealth of gritty detail to shooting things and grabbing people, if you're into that kind of thing, and even with the basic magic system, basic psi, a magic-as-advantages approach, RPM, and variants to be found in Thaumatology, there's always room for new approaches to extra-normal abilities.
No, what I'm talking about is finding topics on which new, general rules can be written where none existed before, or broadly applicable rules for notable adventuring topics. GURPS is densely written, so it takes some unpacking, but there's an astonishing breadth there. For example, fighting, as a general thing, is covered perfectly well in the Basic Set. Might some people want more for a campaign emphasizing detailed combat? Sure. Martial Arts and all that aren't necessary, even though they're pretty nifty. Infinite new magic systems could be written to reflect various views of the supernatural, but magic as a thing already has more coverage than I'll ever use. Social interaction? Social Engineering provides lots of welcome detail, but Basic's treatment is really quite good for the few pages it covers. Basic likewise has reasonable coverage of common physical feats like swimming, climbing, lifting and throwing things, and so on. Chase scenes? Emergency repairs? One character helping out another using a related skill? Action 2 has you covered. Bestiary? OK, there aren't a lot of general rules for making beasties, but there are plenty of examples scattered across various different books (some might take exception to the products on offer and their organization, certainly, but there's a significant amount of available information).
So what's left, in terms of general rules? Not a lot. When I'm confronted with an adventuring situation, it's rare that I can't find at least some applicable rule. There's room for some general design systems, notably for vehicles, weapons, and armor. I think there's also space for GM-centric world-building rules. City Stats is an excellent example. Possibly something similar could be done with environments. It'd be interesting to see generic rules on transportation and trade networks, helping the GM come up with everything from travel times and costs for PC travel to plausible markups for imported goods to adventures based on the act of going from point A to point B. Mostly, though, there are focused rules dealing with specialized circumstances and vast room to expand on the system with worked examples based on the already-solid basis the existing rules provide.