I would love to see this happen but the downside is it doesn't seem like Infiniti is ready to go this route, instead they're most likely to stick to the Q50 and improve on it. But fact is there seems to be room for models like this, after all look at BMW, Mercedes...then there's Hyundai that might be coming out with something.
I think Infiniti's lineup does a good job of covering of covering the sedan market. They have the Q50/Q50 hybrid and the Q70 for a more luxurious feel. they're more likely to come out with a new Q60 convertible than another sedan.