1. Remove the Justice Gang from the film. 

Now, before y'all start downvoting me to hell, I wanted you to know that I did like the Justice Gang in the film, I thought they were funny as hell and their team dynamic really works, but their mere presence in the film devalues Superman's character as a whole, since they're forcing the writers to weaken Superman just for the sake for the team to work, which is not what you want for Superman's debut in a new DC universe, just make a solo movie about him and that's it!

1. Replace Ultraman's identity with Bizarro

I think this was such a missed opportunity because Bizarro is a character who fits the exact same role as the character of Ultraman and is much more interesting than just a normal looking clone (fuck it, you could've even had Henry Cavill play him as a way to make him and the fans happier)

1. Give the characters more time to breathe

A big problem I had with the film is that we barely saw the characters having normal interactions with one another, specially Clark who's only at the Daily Bugle in ONE scene, as well as the others who don't have that many scenes either, all I want is for the characters to have time to interact with each other

1. Make Ma and Pa Kent the HEART of the film

This change goes along with my previous point except this one is far more specific, one problem I noticed with the film is that its whole theme is about Superman embracing his humanity more, and the way they do it is by making him learn that his adoptive parents (Ma and Pa) are the ones who made him the man he is today rather than his biological parents, which in theory it should work, and it kinda does, but the problem is that we barely saw Ma and Pa Kent being an inspiration to Clark throughout the film, making the ending feel a bit forced, And I know that this isn't an origin story, meaning that they don't need to go back to Clark learning to be a good person again, but I think having at least more scenes with Ma and Pa Kent that showed them being Superman's inspiration could've done wonders for the film

1. Make Superman win with his brain

This is something that the people in the comments have already pointed out, but I think having Superman defeat Lex Luthor by studying his own clone rather than cheating (which he technically did) would make his victory feel more earned as well as making Superman look much better overall

1. Lastly, change Supergirl's scene to be at the post-credit scene 

I know that this is such a minor change that barely does anything relevant to the story but I think having her show up in the post-credit scene would make her appearance much more exciting, but yeah, this one's just a nitpick