The Didcot stoppers were the Oxford stoppers, before the wires fiasco, so I would expect they return once that part is extended. EWR terminating at Didcot could be interesting, as it offers connections from the West - but not as many as Reading, and without XC/SWT too. So it would still feel incomplete in that regard.
Appreciate Cowley is not funded or official, but equally, it could be completed much sooner than EWR or wires, if fired up. Not to mention Oxford station itself. Chiltern have been mentioned as most likely, but I'm sure it will all change.