I consider there to basically be three rules for making a good Trek series:
1) Tell a good, entertaining story.
2) Exploration is about space, but is a metaphor for the exploration of humanity.
3) Maintain believability and consistency.
Now those are pretty simply stated, but I think those are
the three rules that TPTB must adhere to beyond everything else. Fears of maintaining the franchise and the Trekkie fanbase while attracting new viewers won't be a problem if you instead focus on those three goals first and foremost.
I see TNG as a sequel to TOS, in both theme and substance, especially after having read the Phase II bible. Indeed, I think I prefer that TNG was made instead of Phase II, because to me, Phase II felt like a sequel series rather than TOS Part II, the latter of which I personally would have wanted if the old cast was being utilized. Despite being a fan of Jon Povill, I'm not completely confident in the overall quality of Phase II, and I'm relatively confident that there would not have been succeeding Trek movies/series afterward. Speculation, of course.
Do I think that the TNG production successfully pulled this sequel-to-TOS thing off, though? Not completely. De-emphasizing the "boldly going" bit was definitely a shame. I mean, the spirit was there, especially early on, but I think one thing that most people don't realize is that TNG changed the franchise's focus. I think on TNG the characters and universe of Trek itself began taking precedence over the actual stories, which I ultimately conclude was a mistake.
Now, I think this is where I see the franchise differently than many people: to me, DS9 and VGR are
spinoffs of TNG rather than sequels, a spinoff being defined as a show that is created from an original, similar show sharing the same "universe" but with a different, specific goal in mind. I'd say "Law and Order," a favorite of mine, is a good model for franchise comparison. True, DS9 was launched from TNG, and VGR from DS9, but I'd still argue that since the latter two were both set in the same universe of TNG, both were spinoffs of TNG.
Therefore, I'd have liked to have seen TNG as the show that carried on the "boldly going" spirit of TOS, and really went out there. Leave the inter-UFP political stuff for DS9, since they did that so well anyway.
Some have also proposed before the "ER" approach to TNG - that is, write out new characters as old actors want to leave, promote everyone left up the chain, and bring in new characters to fill out the ranks. Personally, I'm fine with this. "Law and Order," again, has done this pretty well IMO. I would honestly have not minded if TNG continued to run concurrently with DS9 and VGR so long as the three shows presented distinctly different types of stories. Like with "Law and Order," there are certain stories and formats that each show would focus on, and the others would avoid doing, but they are all, generally, about the same thing.
I think, also, that taking TNG to the movies was also a mistake, especially since, again, what was offered in the TNG movies was not the same as what was offered on the show. I mean, they blew up the ship
because they wanted a new one! Compare that to the IMO emotional scene in TSFS and to me the TOS movies are still light years ahead.
There aren't very many changes I would have made to the original cast of TNG, but there are a few big ones. I would have made LaForge the engineer from the start, and Worf the pilot. I would have fought to kept Yar, and generally try to flesh out the characters more in the first season. I would have also put Data in blue, but keep him as ops officer, as a sort of quiet acknowledgment that he is the thematic descendant of Spock. Wesley would have never been an acting ensign - he might have had a job on the bridge, doing the sort of "back of the bridge" stufff Worf often did.
My main changes would have been in regard to Troi. I always felt that she was a great idea, and I like Marina Sirtis, but she was just misused in every way. I'm of two minds about having her as a Starfleet officer; if she is one, she needs a uniform. It can be a more female uniform (read: dress) but should not be the "bunny suits" that she was so infamous for. I would have dropped the empathetic angle, but kept her an alien. She would have a doctorate in psychology, and still be the daughter of a famous ambassador. I would give her more of the Spock characteristics; she would be the one who possessed the logical, analytical mind (while Data would remain the encyclopedia) and would advise in matters of first contact and general well-being of the overall crew. I'm tempted to make her a civilian advisor of some kind, but I'd hate to make her a Soviet Politlcal Officer.