With the coming of Cowley and the additional running line to the south from that, I would look to try and segregate the Marylebone and Paddington routes as much as possible.
With the constraint that nobody is going to be rebuilding the Botley Road bridge again, I think the general shape of things as they are could work out. Build platform 2 through, and by moving the buffers south a bit, the track accessing P1 could feed 2 north facing bays for EWR of ~175m.
Then throw some money at resignalling things so that to the north of the station the western pair of lines are for the Cherwell valley and Cotswold lines, with the eastern pair are for the OXP route. To the south the western pair are for Didcot with a single line to Cowley on the eastern side of the formation.(I'd add also a station with a passing loop at Kennington/ Redbridge P&R and platforms on the Didcot lines).
Platform 4/5 become the platforms for Paddington/ Birmingham/ Worcester and 2/3 are used for Marylebone/ Cowley, with reversing sidings on both sides of the formation for trains that terminate from the south, or they could carry on to reverse at Hanborough/ Charlbury/ a new station at Kidlington.
The rest of the "Reading style" rebuild is rebuilding the station building that I've demolished for the new platforms.
