Can't see it happening personally, though I do think trams would be good for Oxford.
St Giles to the P&R is too much of a gimmick, I don't think it would generate the required custom. Presumably the route would be via Banbury Road, which would service North Oxford and Summertown which is a fairly busy bus route, but I would expect quite a lot of resistance to digging up the road for months. Alternatively following the railway somehow would be easier (except reaching it from St Giles), but hopeless in terms of connecting anywhere.
Using the Cowley branch makes sense but isn't going to be all that useful unless trams run on-road at least to Between Towns Road. And at the town end, the route would need to run in from the station as well. The prime bus corridor in Oxford is down Cowley/Oxford Road (1/5 route)- this is mostly narrow and congested though so not very suited to trams.
I've thought for a while that Abingdon would be a good target (making the local politics even more complex), following the rail alignment as far as possible. Most of the original branch would be usable or could be reclaimed at low cost. Not sure how you'd incorporate the new estates to the north though.
In fantasy land (see quote below

), I'd propose Water Eaton - St Giles street running, then underground to link to the rail alignment just at the car park to the south of the station with stops at Bonn Square and the railway station, then following the railway to Cowley/Blackbird Leys (with some street running either end), and Abingdon (continuing into the town centre at least).
TFA said:
And why couldn’t we create a monorail connecting key locations around the city’s ring road?
Haha, I needed a good laugh!
PS: also, the trams in the presentation slides are
hideous!