Why you shouldn't say "will rather"
What kind of error is it?

Grammatical error involving misuse of modal verbs
Why do people make this mistake?
People sometimes confuse the modal verbs "will" and "would" when expressing preference. They incorrectly combine "will" with "rather" to form "will rather", thinking it conveys a future preference. However, the correct expression to indicate preference is "would rather", regardless of the tense.
What is correct?
The correct phrase is "would rather" to express preference or choice between alternatives.
Examples of correct usage
- I would rather go for a walk than watch TV.
- She would rather read a book than play video games.
- They would rather travel by train than by plane.
- He would rather have tea instead of coffee.
- We would rather wait here until the rain stops.