I'll go first.
Have the show have more focus on plot from the start instead of how SoL it was in the first season. Season 1 is very different from the rest of the show so I'd like to make closer to what Season 2 was with it's focus on plot for more consistent pacing.
Focus a lot less on Earth characters that were mostly irrelevant to the plot like the ones at the karate dojo.
Remove Ponyhead from the show entirely. She's annoying, adds nothing of substance to the story, and makes every episode she's in worse.
Have Kelly be introduced in the first season in the role of Star's best friend instead of Ponyhead.
Abandon Starco. Enough series as it is have the main male and female character get together in the end. Having them just remain friends by the end would make for something more interesting. Probably going to get a lot of shit for that one.
Have Jackie no longer be a crush for Marco. Instead have him be more of an admirer who thinks she's really cool. I wanted to remove most of the shipping drama from the show so unfortunately Jarco is a casualty of this.
Set up Kelly as Marco's endgame relationship early on with them eventually getting together midway through the series.
Keep Tom's arc more or less the same and have him and Star be set up as endgame.
Have Jackie turn out to be a mermaid with the ability to control water That way when she starts going on adventures with Star and Marco she'll have something to bring to the table.
Have Janna remain mostly the same, but increase her involvement in the plot and eventually have her learn to use magic of her own.
Have Ludo's arc play out similarly, but have more time devoted to episodes about his redemption and eventually have him return to the story as an ally to Star with his own magical abilities he learned while in exile.
Toffee instead of being killed in Battle for Mewni is horribly disfigured and goes into hiding.
I'll continue in another post.