Just having a bit of a "if I ruled the world" moment right now. I'll list a few examples of bad or mediocre shows, and some small details I think would have helped them out. Maybe not turn it around, but it would have been nice. Whatever. Follow up with your own examples in the comments. Please, no "I'd never make the show" things unless you're talking about The Matrix sequels.
Star Trek: Voyager: Slowly kill the pristine nature of the ship. Without Starfleet repair docks, the crew should have made finicky replacements from Delta Quadrant spare parts. Have them restock some of the crew with curious/adventurous locals. Not just Neelix and Kes. Having a holographic doctor may be spiffy, but you'd think they'd at least supplement him with an alien doctor or two who'd study up on Alpha Quadrant physiology to eventually replace him. Not Tom Paris.
The Big O, Season 2: Kill all the bright colors. Kill the Gainax Ending. Dump Big O's transforming arm cannons. It won my heart with clumsy, oversized fisticuffs. Not exactly a small change on the ending, but pleh.
A number of Gundam series: Dump the super-prototype angle. Give the heroes standard mechs with some customization, or at least new models that only have marginally better combat capability compared to the local Zaku. I know the toymakers like to have a half dozen or so flashy, spikey new models to make with each new incarnation, but come on. If you must insist on several new models, have them each fill different combat roles (but be wary of making an Aquaman), rather than just being bigger and badder than everything else. What should matter is skill, teamwork, and planning.
That's all for the time being. Feel free to list your own suggestions for series that could use touch-ups like that.