I am interested in adding this feature to my existing color circles extension.
Adding the circle to all discussions, just like it's on all posts is the easiest implementation.
Adding the user badges to the discussion list is also possible, but one warning here: if extensions define custom badges that are not linked to groups, they will probably not render in the same way, same order or at all in the discussion list, which could lead to confusion. Also, what should be done if a user has many groups ? The UI of the homepage won't be suited for that.