Sadly most likely not. You will have to wait until the FoRS retires though, it has been said many times over 1 RS at a time. The worst part is I've heard more rumors about a RS SUV then a FiRS which is kind of depressing. Ford also stated a while back that they intend to build more RS vehicles and what they mean is instead of having a 7 year spread it may only be 4 between new models.
Good news, build your own. The parts are available to take the FiST to the next level while being a reasonable build. I paid 20K for mine and I honestly think 7k would get me there and besides things like interior differences or fancy stability modes you will still have better parts then what a FiRS would.
Other bad news, Fiesta sales are ok but not amazing. I also just received a notice from Ford saying they are ready for the expanding SUV market over the next 5 years with new models coming out. So there seems to be a change in the long term market and smaller cars are fading out again.
Could it happen yes, but I wouldn't hold you breath. I bought my FoST back in 2013 anticipating the FoRS. I figured 3 years and trade it in. Well back in 2011, the FoST came out a year later then I anticipated which I was following to purchase as a filler vehicle. Eagerly followed RS development before it was even a mainstream thought, when I had the FoST, when the RS finally came out took over a year to get here and extremely difficult to find a dealer to sell at MSRP, not to mention the horror stories of dealers backing out when the vehicle finally arrived. The other bad news is that unlike the FoRS the FiRS could potentially be Europe only, or extremely limited, a very likely possibility that needs to be considered. Either way, I'm not waiting for it and I'm not making any decisions based off the possibility it may come out.