The amount of torque militates against the need for six gears, and the practicality.
I would also like the final gear to be higher, but you can do it just by stretching the distances between some of the other gears, notably between fourth and fifth.
Because of the extreme low end torque, the bike would still pull fine and not have to be revved between gears, so there is really no need for the extra gear.
Also because of the extreme torque, the gears need to be a certain width, so the extra gear will add extra weight and bulk to the gearbox, or will result in a less robust transmission.
Torque probably wasn't a big problem on the 500 Kaw.
I could live with just a higher fifth gear. If they wanted to do it right and change the other gears to optimize things that would be nice. I think that a sixth gear would be a bad trade-off, engineering wise.