I'm a relatively new fan, but I've watched some performances when Sunghee was still with the group. She really has a wonderful voice.
As much as I think the group would benefit with her addition however, KARA would have to evolve once more to add her in, the same way they worked hard when she left and JiRa joined. Maybe they could do a song like KARA feat. Sunghee ... I think that would please a lot of the fans who's been with the group since the start.
But then, I don't think Sunghee's planning to come back even as a solo artist (I think I read it somewhere). If she did go solo, I wish they'd do that