I believe access to platform 5 blocks a parallel access to platform 4. Perhaps someone is able to confirm?
If (when) Doncaster -Sheffield is electrified it would make sense for Sheffield -Doncaster trains to be electric and probably go though to Leeds. Scunthorpe to Sheffield stoppers provide a through service from Thorne, Stainforth and Kirk Sandall which SY PTE is likely to insist on, given they provide funding. The PTE also desire a through service Meadowhall to the Worksop line, though this doesn't need to be a Doncaster service, it could be from the Barnsley line as it used to be when Huddersfield trains went to/from Lincoln. The timings on that service were too tight to be reliable, but maybe the Leeds via Castleford might work, or the semi-fast?
Given the regular delays on platforms 4 & 8 at Doncaster, often caused y late running East Coast services, I would have thought a new island platform on the goods lines (one face or two) would be good for Northern's locals, leaving 4 and 8 for EC, XC, Hull Trains, TPE and Northern's fast Hull-Sheffield.