Everything I heard about TNG is that nothing ever was planned but the kept story ideas up in the air and maybe would come back to it. Voyager was the first one to try a serial arc with four episode with Paris in Season 2 and DS9 really ran with it and succeeded with six episodes in Season 6. I remember what a huge deal both of those events were. Ira on Shuttlepod Show talked about what a big deal doing a three episode arc was for Season 2 and yet they're saying they were going to do an eight episode arc before that?