I'm not so sure there will be that many incremental changes made to chips once they start to hit the process limit. I say this as I expect the margins on chips to become so slim that it would be financial ruin to invest in the NRE for another 28nm chip once you already have one.
I guess it all depends on how "good" existing designs are. Taking HF just because they provided die size, hashrate and power info.
One 18x18mm die is able to do 400 GHash (nominal - more overclocked**)
Hashing per square mm:
18x18mm = 324mm^2
400 GHash / 324mm^2 = 1.23 GHash/mm^2
Reported power consumption (at the chip) is 400 GH / 250 W = 1.6 GH/W.
So HF claims their silicon is good for 1.23 GH/mm^2 and 1.6 GH/W @ 28nm.
While they are impressive compared to 65nm tech it remains to be seen how "good" those are compared to what is possible for 28nm tech. Are they very good, or barely adequate in the grand scheme of things?
If the run some simulations on an improved die and it shows to only be marginally better (say 1.4 GH/mm2 and 1.8 GH/W) then I agree the chip probably won't be made before moving to a smaller process. On the other if they came up with say >2.0 GH/mm^2 and >3 GH/W) it may make sense to produce a second 28nm chip. I just wonder how much improvement is possible, I guess once we get a couple of vendors with real 28nm silicon we should have a better idea.