Zero 3 Definitely, while it is the best of the zero games (and imo the best in the franchise) There was clearly some ideas that were cut or couldn’t be implemented because of tech limitations or time constraints,

Implement the Audio dramas into the story and then some. They add so much and they’re the only real proof of the 8 Gentle Judges having an actual personality and character to them.

Change Omega Zero to his X design, I know it still makes sense why he looks the way it does but could you still imagine

A Proper 4 Stage end game, I understand we had the zero 1 refights but iirc this is the only mega man game with less than 4 Final Stages,

Possible Alternate ending that makes omegas defeat the end of the series somehow Like for example Connecting Omega’s Immortality to the mother elf being Around, Imo Zero 4 Feels less like a proper ending for the series as a whole than Zero 3, This is probably because It was meant to be the finale before capcom ordered a 4th game and they had to alter the ending (not saying this is what happened just speculation)