Too bad he was never given an opportunity outside of Star Trek to create even more cool stuff.
It may be a case of quibbling over definitions, but I believe I can site at least one example when he did.
The Leif Ericson Galactic Cruiser, later repackaged in "glow in the dark" plastic as the Interplanetary U.F.O., was an "original" plastic assembly model kit released by AMT. The company commissioned Jefferies to design that craft. It was supposed to be the first in a series of space opera type kits that AMT would own entirely. The kit company would not need to pay royalties.
Alas, the kit did not sell as well (during its first release) as AMT had hoped, so no more kits were released. It's possible someone at AMT kept other doodles Jefferies drew, but nothing else was officially negotiated.
Ironically, when reissued as the UFO, its image was often printed upon the boxes of Trek prints and its last box art design matched the scheme of the Trek kits. Some kids (well, I did, anyway) used it to represent an alien craft the Enterprise might encounter.
But, yeah, Jefferies did do some SF designs outside of Trek, though that particular instance got unofficially "assimilated" later.