Based on everyone's feedback so far, here's my latest version.
Extra Attack
Prerequisites: 5th level, proficiency with one martial weapon
When you use the Attack action on your turn, you attack twice instead of once.
What changed?
- Removed the scaling with level
- Only triggers "when you use the Attack action." Can't be used with spellcasting, can't be used as part of a Reaction or Bonus action.
- "you attack twice instead of once." Thus, if you are already making two or more attacks, this feat does not confer any benefit.