The platforms at Sheffield are long, so some building out could presumably be done with both 1 and 2.
Platform 5 is not much use at the south end because departures moving over to the up line need to use the points well north of the south end. The old lengths worked for HST so presumably with the 810 it would only be an issue for 10-cars.
Platform 6 does not seem to work as a terminating platform for trains from the south, so could be "built out" to leave a bay at the north end.
Platform 8 has a bend in it, but I think the geometry is hard to restore the old layout and place a bay at the north end.
All of those extra north facing bays would presumably be useful for the timetable.
Requires a lot of pointwork though.
A design that had more through platforms might be better, but that would mean demolishing some of the islands and I can see why that would be less popular. However, like Cardiff Central, the needs of the operational railway should take precedent over the architecture from a past operational need.