making an undertale game

[iyenal]

Your objective seems to me abstract.
I have the impression you are telling this : "i'm making a undertale game but what I have to make ?"
1 - Try brainstorming to clarify your ideas
2 - See what's your competences to know where to focus
3 - See the original game, and search an "added value": why the user will play with your game and not the original Undertale ?
4 - Sketch on your paper what you want that your game resembles

Hoping I have helped you.