Probably a good way of increasing outer commuter capacity into Birmingham. Oxford is just a sensible place to run to.
Quite possibly. But I maintain that it would be better to look at this properly to identify where the need exists and then provide the services that meet that need.
To me it makes sense, the long distance traffic should go via Coventry (and if could be pathed, a second tph should go that way) - but Oxford is a regional centre and I could see a < Oxford - Banbury - Leamington - Warwick+/Parkway - Dorridge - Solihull - Moor St > service, priced at a regional/commuter level, vs IC structure - being very popular and useful both ways. Possibly an Oxford - Stratford a few times a day too.
And then maybe XC could run fast from Oxford to Coventry on one hourly service, if a second returned.
XC via Coventry is vulnerable to delays and is already a significant issue. If you were going to run the second hourly service that way, it might as well make all the stops as it won't be any faster even if it only stopped at Coventry. Also, it's worth noting that running a fast/semi-fast service on a 2tph frequency would
reduce XC's capacity along sections of the route, in which case you'd definitely need something extra making the trip to make up for the lost seats. 2tph via Cov would also depend on whether the second hourly path even exists along that section or whether that is currently used by LNR/Avanti. It's also going to require an additional path along the single line through Kenilworth. Granted there's not many passenger services through Kenilworth, but there's a decent amount of freight going that way.
Someone else more familiar with Chiltern's operations will need to address the feasibility of the rest of your plan. However, I do like your idea of an all-shacks Stratford to Oxford service. Your mooted Oxford-Brum calling pattern wouldn't actually add much between Oxford and Leamington Spa if taken together with your idea for changes to the XC service, as you'd still only have a 2tph service using single sets calling at principle stations, which is precisely what XC were doing pre-covid.
It's not the greatest of services from GWR, a train every 90 minutes to the intermediate stations. For Kings Sutton, a journey planner often seems to suggest going to Banbury on XC then doubling back with Chiltern or GWR.
Yes, I'm aware. Clearly the greatest demand from Kings Sutton is towards Bicester/London
For Oxford, it's mostly just a general need for more trains heading North. Reinstating XCs second service in the hour would be a good start, though presumably would come at the price of reverting the current double sets back to singles. Birmingham offers connectivity to a wider set of destinations without having to go in the wrong direction first to London.
How many of the London trains terminate at Oxford? I appreciate that some continue along the Cotswolds line, but I know that some terminate and shunt into the Down Sidings before returning to London. Would it not be a better option to extend a few of these to Banbury? There's also more than just a few of the Didcots that terminate at Oxford which could be extended.