I've always thought that getting GWR to Banbury would be a help with capacity on XC as there's not great connections with the XC services at Reading from some locations. Add in direct services from Slough and the like and you could find that a good number of people opt to avoid XC services.
However, with that XC would likely benefit longer term as journey opportunities would be better meaning more people are likely to be using the trains and so when they do have longer trains, although there would be a share to give to GWR, they would still likely end up with more money overall.
All Crosscountry services from Bournemouth via Reading travel through Oxford and Banbury. From there XC service either run through Coventry, or Warwick to Birmingham. At Banbury, Chiltern are the primary operator. Depending on the Chiltern train you catch, the service can be anywhere between three and fifteen minutes slower than using XC from Banbury. However the flip side is that Chiltern are usually far less overcrowded.
If GWR ran to Banbury it would allow people from Reading and Oxford to get to Banbury, then allow the use of either XC or Chiltern to Birmingham, and on-wards. Yes, Chiltern run to Moor Street, however this is a short walk from New St Station. If GWR was extended to Leamington Spa, even more options would exist through West Midlands Trains.
The only other option is for XC to invest in a class 802 fleet. The current situation won't be tolerated forever, especially when some passengers (including myself), do everything possible to avoid using XC.