Throughout TNG, the show had characters come and go, but the main cast remained the same. What if the show had been more dynamic, and cast members and their characters had come and gone in a much more drastic way? Whenever an actor got bored, no longer worked, or their salaries became too high, a new character and actor would replace them. Something more in the Dr Who style. An actor leaves, their character is replaced, and the show keeps going into infinity to the point where it becomes an entirely different cast several times over? This is not unprecedented in the history of the show or other Star Trek series, but it would take things to a different level. Denise Crosby was unfulfilled as Tasha Yar, so she left, and her position was replaced by Worf. Maurice Hurley, when he ran into disagreement with the cast, threatened to blow up the Enterprise-D and everyone on it, and build the next season around the tragedy and building a whole new Enterprise and crew. I think if it were a gradual thing, it would be feasible. If it were a total replacement, fans would riot. But if one character were replaced, and then a few years later another one, I think it would be a possibility. And it would address issues like actor salaries or characters getting stale and writers having nothing more to say with them. TNG could continue on forever like that. It could even transition to a different Enterprise or an entirely different setting, since this is one of only two Star Trek series not tied by a name to any one ship.