Honestly, it's probably for the best.
I'd be damn curious to see it though.  No way in hell could Murtaugh still be an officer, he'd basically have to be retired and for the plot to be something a little more personal to have him on some crusade, like rescuing his daughter in the original or the refugees in 4.  Guess it's not out of the realm of believability for Riggs to still be working though at his age, upper-brass desk job or something.
Man I would kill to see Shane's version of Lethal Weapon 2 though, "Play Dirty".  The bare bones of it sounded the same, South African apartheid guys as the villains and all, but apparently it was really ****ing dark and intense even by the standards of the first one.
Some broad re-working of the themes of that could be cool, obviously have to swap out the Afrikaaners for someone else, re-tool the idea heavily, but keep all the vicious intensity it apparently had.  You could finally kill off Riggs as intended too.
But yeah, always been skeptical this'd ever happen.  The fourth works as a really nice satisfying send-off anyway, it's not going to be a problem if that's where they leave it.  Then again, Rocky Balboa was pretty amazing too and then they came out with Creed, so it's not like a kind-of-unnecessary continuation can't turn out awesome.