Besides, I would be surprised if they were able to make money with it below a 1,700~1,800 US$ price point.
The best maybe to look at it as a light and very affordable 200mm f2.0.
Cheers,
Bernard
The 135/1.8 already does that, and potentially a better job of it.
It has the same 75mm physical aperture size as the 105/1.4, so will give the same degree of background blur when the field of view at the focal plane is the same (although, as a longer lens, you'd be standing 30% further away to achieve the same field of view). Additionally, at f/1.8, it allows you to keep more of the subject's face in sharp focus than the 105/1.4, while still blowing out the background to the same degree.
Of course, this says nothing about the quality of the bokeh. It may well be that the 105/1.4 has smoother bokeh than the 135/1.8, despite the quantity of background blurring being the same. In that case, I hope it can take a 1.4x TC and has enough central sharpness to carry it well - a 147mm f/2 option, with super-smooth bokeh, could be even better than either 105/1.4 or 135/1.8.
I'd like to see Sigma bring out a 200/2 lens. It would be the next logical step in their lineup of portrait lenses, and given recent history, would likely give the Canon and Nikon versions a run for their money.
