More Vulcans (eventually one episode which orbits around Spock and the progress of the reunification). Maybe a continuation of Conspiracy where we learn more about the creatures. Of course, at least one or two Worf episodes is obligatory through tradition. Likewise with Q.
And I always wanted TNG to rip off TOS more dstinctly. Such as a mirror universe episode, premature aging (exactly as we saw it in The Deadly Years), Picard or Riker put on trial for either the death or "death" of a crewmember. I'm uncertain if someone would really die and that it's just proven that it was not anyone's fault or if there would exist some kind of vengeance factor and that the person would either fake his death or give his life for the cause. The representatives of judiciary would come onboard the Enterprise and hold the trial there instead.
There are some interesting unused stories from both TOS and TNG that were never used that I like.
The first one is a Q story that was intended to be in Season 5 of TNG. From Memory Alpha:
In "Q Makes Two", Q would have duplicated the Enterprise and the crew according to some uniform characteristic. Brannon Braga recalled, "There was a sense of doom from the moment we started 'Q Makes Two.' I think we broke it three times. René wrote two drafts and it was ultimately abandoned. It's an interesting notion that Q comes on board and Picard's saying people are inherently good and we have managed to get rid of our darker elements in the 24th century and we're better people. Q says, 'So you don't think you have dark components and you think you're better without them, well I'm going to show you a thing or two,' and so he extracts the darker components and puts them into doubles. The clean, good components suffer and so do the darker components and neither functions without the other. We see that dramatically, but for some reason we made it more complex than it needed to be. It's a show that could still work. The image in my mind that we never really got to was the two Enterprises shooting at each other, that's what you want to see."
Jeri Taylor added, "'Q Makes Two' was a debacle and it plunged us into a nightmare of having to get "Man of the People" ready." (Captains' Logs: The Unauthorized Complete Trek Voyages)
According to Taylor, the idea of splitting a starship in two would later inspire the Star Trek: Voyager episode "Deadlock". (Captains' Logs Supplemental - The Unauthorized Guide to the New Trek Voyages)
The plot also notably resembles the original series episode "The Enemy Within", except with the entire crew duplicated, rather than just the captain.
As a Spock & Sarek fan, I of course would love this idea simply for the sole reason of seeing them on my screen. I do think the plot would need some developments/interpolations though.
A story that D.C. Fontana and Herbert J. Wright pitched, during TNG's first season, would have featured Spock, Sarek, and Romulans. Fontana detailed the specifics of the proposed plot, which had some similarities to the ultimately-produced two-parter "Unification I" and "Unification II"; "We're taking on a mysterious Vulcan visitor who, of course, turns out to be Spock, and his mission is to rescue his father, who has been captured by the Romulans while on an exploratory peace mission. Now he's being held hostage, and they want Spock." Fontana was told that the plot was not granted production approval due to the unlikeliness of obtaining either Mark Lenard as Sarek or Leonard Nimoy as Spock. (Star Trek: The Magazine Volume 1, Issue 2, p. 87) According to Herb Wright, the reasoning was that Nimoy had "a falling out" with Gene Roddenberry and instead directed Three Men and a Baby. Wright noted of the story, "I thought [bringing Spock back] would have been great first season." (Cinefantastique, Vol. 23, No. 2/3, p. 48)
There are also many TOS stories that could easily be accommodated to TNG. BTW, there should be a "Seasoning Season 4" in the TOS forum.
I especially like this one and I always found this story more suitable for TNG rather than TOS. Especially considering that the conceptual emanations are somewhat cognate with Angel One.
The World of Star Trek revealed a story premise that DeForest Kelley had always wanted to see, featuring himself and Nichelle Nichols, described as "something where the two of us were thrown onto a planet where there was a great racial problem, only reversed. The fact that I am a Southerner and she is black, and that we're trapped on this planet together."
Stephen Kandel described the story as "a show with a very militaristic planet being discovered, and the inhabitants were black. The only one they would talk to was Uhura, and they regarded the others as nothing." (Starlog issue #117, p. 44)
In The World of Star Trek, David Gerrold added a footnote to DeForest Kelley's premise, stating, "As a matter of fact, the idea was one that very definitely had been considered. A script version had even been written. And rewritten. And rewritten. The story involved a planet where blacks were the masters and whites were the slaves, but either the premise was too touchy for television or nobody could quite make it work. The script never reached a form where Roddenberry or Coon wanted to put it into production."
The basis of this episode was probably a story premise in Roddenberry's 1964 proposal Star Trek is..., entitled Kongo, about a planet with the "Ole Plantation days" with reversed roles of blacks and whites.
Stephen Kandel referred to the fact that the story was not given the go-ahead as "unfortunate" and went on to say, "I would love to have done it." (Starlog issue #117, p. 44)