Such requests usually fall through when the ‘head of the project’ have zero ROM hacking experience.
Maybe try something basic, like using the tool “Advance Map” to edit maps and have NPCs in different places, changing the encounters, learning how to make working warps etc. After that, you can move forward with scripting, which changes what the NPCs say and do, custom encounters.
And then past that, there are other tools in the “Advanced series” that can help you change how the sprites look. Should be easy to google.