I would just try to keep the width and height of the new band similar to your existing band measurements. And diamond color close too. Go to jewelry stores and try on a variety of bands, if you already haven't done so. In the meantime, check out the stack of the day thread for inspiration, the variety seems endless:
You could try getting CZ samples to see if you like it, I think you could pull it off. I saw this insane stack while at my son’s swim class. The woman had a huge halo radiant or some kind of rectangular cut stone with at least 3 or 4 bands stacked with it. You couldn’t NOT see it.