I think a guest appearance from Michelle Forbes would have been great. Not a regular, but maybe just a one-off return where she and Kira clash over different ideals. I'd love to have seen that.
But I wouldn't trade
Kira for her. As others have already said, Kira Nerys is a much more dynamic character than Ro Laren. The fact alone that she isn't
Starfleet makes a big difference (yeah I know, technically Ro wasn't by the end of TNG either). One of the things which makes Kira so great in the DS9 cast dynamic is that she's got her own life and largely her own agenda. She isn't tied to the uniform, but when she does
eventually get to be part of Starfleet it feels like a logical progression for the character. Ro, on the other hand, was always
part of Starfleet and despite her firey attitude she still had to follow orders. Most of the time.