Work began on TNG in early 1986. (And even earlier if you count Paramount's offers to Bennett and Nimoy, and the series that Greg Strangis wrote a bible for that was ultimately scrapped.)
Wait a minute. There was a series before TNG (and not Phase II) that was ultimately scrapped? Any details on this? I've never heard of it.
There's not a lot of details on the pre-Roddenberry development of
Star Trek: The Next Generation. It gets mentioned in both of the Roddenberry biographies, Joel Engel's unauthorized
Gene Roddenberry: The Myth and the Man Behind Star Trek and David Alexander's authorized
Star Trek Creator. Interestingly, it gets more mention in the latter, as Alexander quotes at length from Roddenberry's dismissive letter about the series bible.
Paramount approached Harve Bennett about creating a
Star Trek television series. He said no.
They approached Leonard Nimoy. He also said no.
They approached Greg Strangis, who would serve as a creative consultant on
TNG and later create
War of the Worlds. He said yes, and he was partnered with Rick Berman.
Based on Roddenberry's letter in
Star Trek Creator, what they came up with was set about a century after Kirk's time, and it focused on a group of cadets, not unlike
Star Trek II or Abrams' film. It also involved the Admiralty in some way, which Roddenberry felt was not
Star Trek.
Obviously, the Strangis/Berman series never went into production. I don't know if any ideas carried over from it into
Star Trek: The Next Generation, though it's possible because both Strangis and Berman worked on the first season.
After Strangis/Berman, Paramount approached Roddenberry. Maybe the studio had decided that no one other than Roddenberry could do it. Maybe they were afraid that fandom wouldn't watch a non-Roddenberry
Star Trek series. Maybe Roddenberry's lawyer pulled strings to get Roddenberry dealt back in.
Why isn't the pre-Roddenberry history of
Star Trek: The Next Generation better known? Probably for the same reason that our knowledge of Pelagianism and other early Church heresies is limited to the writings of their opponents -- the history's been scrubbed where it conflicts with the orthodox view of events. What we know of this "lost" series is what Roddenberry wrote when he railed against it, and were it not for David Alexander quoting from that letter, we might not know anything about it at all.