TOS: I would have never come back for a third season. We needed 79 episodes for syndication, I get it. But the episodes in the third season are truly bad.
TNG: I never would've made them a "family." Comradery, sure, but keep it professional. I think they forgot they were doing a job in the last 2 seasons and it ruined the show.
DS9: I would've spent a year making peace with the Dominion. I don't care if that happens in the 7th or 8th seasons, I would've tested the rhetoric at the beginning of the show with showing the slow, methodical work of trying to make peace with an enemy. There was a lot of untapped potential.
VOY: I wouldn't have "stranded" them in the Delta Quadrant. With the Borg and the Dominion breathing down our necks, I would've made Voyager a celebration of manned space travel as we know it would be today. We have an engine to get us out there, not to get us back. We have to grow our own food, conserve our own energy, and other mundane things that would mirror what it looks like trying to leave our solar system. I would've made it a celebration of our adventurous and exploration spirit. Lewis and Clark in Space. No conflict (because it never went anywhere) between the Maquis and the Federation officers. And I would've invested time in making the show original and try to say something. How more interesting would the characters be if they had chosen to leave behind their lives for the sake of exploration? No communication with Starfleet, no map of what we would find, just pure exploration.
ENT: I wouldn't have started the show with the launch of the ship. That has been done in TNG, DS9, VOY, and this show. I would've started the show 2 years before the ship is to be built. The first season would've had smaller missions with our main characters that will eventually end up on the NX-01. I would've toiled with the engine, with the Vulcans, so there is some emotional investment in the show by the end of season 2 when they launch the thing. If you want characters that are more relatable, put them on Earth. Put them on Jupiter, create the new 22nd Earth. We bond with our crew (possibly have guest-star rivals) as they are picked for the mission and mourn for the ones that aren't picked. And each mission is then earned and appreciated.
Just my thoughts, although the DS9, Voyager, and Enterprise series tweaks, I really wish they would've done.