Suppose you were going to make the Extra Attack class feature into a feat. You have your reasons: maybe you love clerics but hate multiclassing, maybe you want to play some kind of weird "half-martial" character, maybe you just like hitting things with a hammer, whatever. You've already figured out the "Why" for your table, and now you're working on the "How." How would you balance it? How would you fill in the orange text?
Extra Attack
Prerequisites: <Your Text Here>
You can attack twice, instead of once, whenever you take the attack action on your turn.
The number of attacks increases to three when you reach 11th level, and to four when you reach 20th level.
I'm leaning toward "5th level, proficiency with a martial weapon" but I'm curious what others might think.