I've never seen the need for a convertible car, but I know that is just my opinion and not the prevailing one. However, both arguments for a higher power version and a convertible version are asinine - this car is selling well enough that they don't need either version.
If I were to put my money on an improvement path that embodied the original concept and the design philosophy, more power and open top are probably the last place I would look. Lower the weight, improve suspension, lower COG would fit the DNA much better. Arguments beyond those ideals are just attempts at justifying your own wants.
If they build a convertible, great. It's a departure from the central concept but it will probably sell well. It just seems to smell a little like a boardroom decision instead of an enthusiast one. Yes, I know the car may have been engineered to be a convertible to begin with, but I still suspect the fixed roof was chosen because it didn't compromise as much as a convertible.