Why are we trying to make XC bigger? Personally, I would suggest removing a lot of the regional services from XC allowing them to focus on being an LDHS operator, trimming the franchise right down to the NE - SW mainline and NW - SW services. If they get paths on EWR via WCML, they can improve that service and add frequency and trains and whatever else.
The reason they have the Cardiff - Nottinghams and Birmingham - Leicesters (- Stansted) is because the 170s are maintained at Tyseley and the drivers based at New Street. Reading - Norwich (sure, why not Norwich), doesn't go near Tyseley/New Street, so it doesn't seem to make too much sense to run it that way.
From a business area perspective, all of the services you've proposed adding to the XC franchise are regional services (not even regional expresses really), many of which could possibly do with either their own separate franchise, or be combined with some other existing franchises who deal in regional services, where convenient.
4 - 5 car intercity units like the 22X series, however poorly thought through, would not really be welcome on any of the more regional services, because they'd not be able to make many of the sprinter speed differentials required, so would actually be slower. They'd also not create any massive capacity increases, because they aren't units designed for capacity, they're (badly) designed for comfort.
Glossary for TLAs:
LDHS - Long Distance High-Speed
XC - Cross Coutroy
NE - North East
SW - South West
EWR - East-West Rail
WCML - West Coast Mainline
TLA - Three Letter Acronym