This is how they should do it. It's unclear how they will do it.
I see three scenarios given BG3's current UI:
1.) The standard "attack" button (that all classes get) is changed for classes with extra attack so that when you click it, you always attack the full # of times. You attack the same target. (Worst option)
2.) A whole new icon "extra attack" is added to the hotbar. When you click this icon (it costs an action), you attack the full # of times. You can target attack multiple people, as long as you can reach all of them without moving, similar to magic missile. (mediocre)
3.) You get an extra attack icon and an "extra attack resource" similar to superiority die but that refreshes per turn (or the button is clickable multiple times per turn). Each click of this "extra attack" icon spends one of those uses. The first click costs an action, but subsequent clicks are free? (good)
Given that other 5e mechanics have been condensed into single buttons (e.g., many maneuvers & sneak attack are activated after a successful hit in 5e, but are chosen before an attack is made in BG3), I'd say that option #2 is most likely.
p.s. there's also the cursed option, where every extra attack you get adds an additional icon to your hotbar xD
I am hoping that they can do it in an elegant way (not too many icons) that can still let us apply proper abilities to attacks (i.e. Battlemaster Menacing Attack X2). I would hate to see games mechanics changed because Larian couldn't figure out an UI issue.
Here's potentially one simple way to do it:
1) Whenever a character with Extra Attack takes an attack action, immediately after, all their non-attack full-actions are greyed out (i.e. spells, channel divinity, etc). All abilities that work with extra attack (i.e. superiority dice) should still work.
2) Some visual indicator should let the player know they have an Extra Attack Action remaining: i.e. below the portrait, the green circle (for action) should be replaced (maybe a white circle for example).* Maybe something non-intrusive around the ability bar to remind you why some abilities are greyed out
* The circles are important to track because you can theoretically action surge after 1 attack, and you'll want a green + white circle to indicate you have a full action + 1 extra attack action. In that unique scenario, your other abilities are no longer greyed out. If you attack again, it burns the extra attack first.
