A lot of reasons prevent this even being something that we'd ever consider, aside from the fact we've done HL2 RP for a very long time; trademark, licensing and other legal issues because we don't actually own any rights to HL2.
Well, can't we do something different (lore/ assets wise) similar but different from HL2? I mean, "Humanity's life / struggle against alien occupiers / Dictatorship government/ Dystopian World" isn't really unique for Half life anyway. We could change the visuals a bit and how assets look and get something really unique and not so different from what we know and love.
The only thing I can't think we can really keep up with is the synths. However, even the term "alien head creatures" or "alien bugs" is not just a half-life thing so we can do those.
(Byt yeah, I get that making something new is Hard, and people would really miss the stuff we can't remake)