Who knows? I'm on the fence about whether Assignment: Earth would've made a good series; the idea had potential, but the actual episode didn't necessarily sell it too well. I've read the script for the original, non-Trek-related '66 pilot, and it was pretty awful. I get the impression it was meant more as a sitcom, sort of like My Favorite Martian but with more spy-vs.-spy, but it wasn't at all funny. The proposal Art Wallace and Roddenberry did for the Trek-spinoff series sounded a lot more interesting. It stressed that the crises would come from human flaws and foibles rather than aliens and fanciful phenomena (which was largely to contrast it with the competition, The Invaders). That could've made it a nice, smart drama, and one with diverse story possibilities, or it could've proved limiting. There was also The Questor Tapes, which was basically a rehash of the A:E premise (cool, intelligent being uses alien tech to shepherd Earth away from self-destruction), but I think it would've been a stronger series, since Robert Foxworth and Mike Farrell had a great chemistry, a lot like Kirk and Spock. I never felt Robert Lansing and Teri Garr had the same kind of rapport. (Of course, if A:E had gone to series, there could've been a cast change -- either a new sidekick or a recast Roberta, without explanation in either case, which was common back then.) Although a Questor series would've been stronger if the movie hadn't already resolved his quest, if there'd been some ongoing mystery about his origins and nature. Then there's Genesis II/Planet Earth. That had potential, and was designed to be a "Hundred Worlds" series like Trek, visiting a different exotic culture every week, just on post-apocalyptic Earth instead of space. Actually this idea was executed elsewhere, in the Logan's Run series and Ark II on Saturday mornings, with limited success. So G2/PE might not have lasted much longer than those. G2 definitely wasn't viable in that form; Alex Cord wasn't an appealing enough lead, the PAX culture was too sterile and unsympathetic, and the look was rather unappealing. The retooled, mostly recast Planet Earth version worked much better, though, and could've spawned a viable series -- although it would've been marred in retrospect by some rather ugly Native American stereotyping where Ted Cassidy's character was concerned. The other Roddenberry pilot that got produced in his lifetime was the supernatural-themed Spectre, which I've never seen, so I can't speculate on how it would've worked as a series. But in David Gerrold's The World of Star Trek, it was mentioned that he was working on an idea called The Tribunes, involving near-future police officers using advanced technology and methods. I've always wondered what that might have been like. Then there was Battleground: Earth, the pilot that was posthumously made into Earth: Final Conflict. I gather the aliens were more unambiguously villainous in the original, meaning it would've been more like V (which was why it was changed).