Would make sense given the outlook of the Cross Country's train fleet with the withdrawal imminently of HST sets. This would probably mean the voyagers running less often as two sets. GWR could take advantage of running hourly services from Reading to Banbury via Didcot with every other hour being all stations.
The Cross Country HST's do not do services through Banbury, Didcot or Reading.
Besides, you will be having Avanti West Coast in the not too distant future replacing their class 221 Voyagers with class 805 and 807 units. So, even though not totally the same as the XC Voyagers, there is the possibility I believe that XC could still use 12 of the AWC class 221's as replacements for their HST fleet. Falling that they could take on the class 222 fleet. But both these points have been discussed in many threads within the Speculation forum.
Also, I doubt even if the paths where given to GWR, that they have the stock to be running the services that you suggest without having to bring class 165/166 units back up from the West Country to be based back at Reading. This would defeat the purpose for which they where sent to the West Country in the first place.