This interests me as well, as I'm considering publishing a supplement for expanded universe and world creation. But despite reading and re-reading threads such as this one, as well as the OGL, I still find it confusing as to how far I can go, or can't. For instance, extended UWP ratings, the expanded UWP string, alternate government types, etc.
I'll continue my work and my research but it's so very disappointing that this situation has become so confusing and seemingly polarized.
The classic UWP and UCP values are open; the design tables that underlay them are not. (T20)
Ships and Vehicles written with T20 are open; the design sequences themselves are not.
T20 Classes are open, as are racial stat mods and special abilities; the fluff-text is not.
T20 Char Gen is open, but the specific services are not.
Mongoose only released SRDs (that I've seen) for MGT1E Core, HG, and Mercenary, and there are some interesting omissions therein. Nothing else is open but what is in the SRDs.
Note that CE could technically be unlawful in some jurisdictions; German and French creator rights are far more expansive than US rights. As in, mechanics are covered, unlike the US (where it's blackletter that processes are not, nor are game rules, but the exact wording, where not essential to the game rules, is) or the UK.
Marc or his licensees could, in theory, block publication to France and Germany under the Creator's Rights laws there. Recent (last 5 years) case law includes a case which involved a german company cloning a french company's boxed card game, with new art, reworded rules, and a slight theme change; they got hit by a french court for full stoppage of sales, and damages, and germany accepted the french decision and applied it there...