Definitely not that branch, if it ever were to re-open. (I think it would work as a light railway-cum-tram though and would favour its development as such, probably not wholly on its original route, at least at the Oxford end).
Why not? (a) it's far too long. The ride quality (and tortoise-like speed) of the PPM is just about tolerable from Stourbridge Junction to Stourbridge Town, but on a journey of over 10/15 miles, it would repel more passengers than it would attract.
(b) infrastructure. The slow speed of these trains is not suited to the busy sections of line between Oxford station and Wolvercote junction, and then along the sensitive single-track section of the North Cotswolds line, before the branching point for the Witney branch.
(c) Capacity. The Stagecoach Gold services between Oxford and Witney run at such a frequency, and, at peak hours, with full buses, that a PPM would more likely than not be inadequate to deal with passenger numbers at busier times. The provision of a high quality bus service that runs from early in the morning to very late at night (and into the early hours at weekends) might well outcompete any train service (not least given how Witney has grown, and that any potential station location there - the former two sites now being unavailable - may not be convenient for many residents of the town (Whereas the Stagecoach Gold routes serve much of the town). Probably a Witney station would work best as a kind of parkway for the Oxfordshire Cotswolds, with a big car park. But where to put it?); at any rate the service would need to be a high-quality offering to attract the relatively wealthy population of the area, and a PPM is hard to sell in those terms.
Now, nearly, if the Radley to Abingdon branch were ever to reopen, yes, I could see that working with PPMs. Or a revival of the Wantage Tramway, from Wantage Road station (aka Wantage and Grove) to Wantage itself. Or the branch from Faringdon to a new junction station on the main line.
Small-scale, short branches without huge custom, in short.