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.
I'd be happy with your example. Although I don't think you're allowed to action surge after 1 attack (when you still have extra attacks remaining). AFAIK, you can only move or perform free actions between extra attacks. As Action Surge costs a bonus action, it would be a greyed-out icon until you spend all your extra attack white circles.
That said, the "white circles" are still necessary as an indicator of how many extra attacks you have left. Hopefully all the greyed out icons would also show
why they're greyed out when you hover over them: e.g., "You must use your remaining X extra attacks"
