When HS2 is open, XC will become redundant.
On the Birmingham - Manchester corridor, fast services will not be required. The services should be taken over by LNWR and run every 30m calling at more stops such as Kidsgrove, Congleton and Cheadle Hulme. They would be operated by 350s and between Birmingham and Stafford, would form 4tph (every 15m) with the Birmingham to Liverpool services. There will be no Northern services between Stoke and Macclesfield with the Northern services starting at Macclesfield. Birmingham to Crewe via Stoke will not need to run as after HS2 is opened, as there can be 2tph London to Crewe via the Trent Valley and one can go via Stoke.
From Birmingham to Reading/Bournemouth, anything from Reading southwards will be faster to change in London from HS2, so services will only need to operate to Oxford. These can be transferred to Chiltern, and both will go via Coventry as there will be only 2tph Avanti after HS2 opens. They will also call at Kenilworth, replacing WMR services between Leamington and Coventry.
Birmingham to Leeds/Sheffield/Newcastle will become completely redundant. 4tph commuter services should run along the corridor from Birmingham to Nottingham (2tph) and Sheffield (2tph) providing Tamworth and Burton with 4tph. These these would be transferred to EMR. Birmingham to Stansted/Leicester has no relevance to HS2 but would also be transferred to EMR.
Birmingham to Bristol/Plymouth/Penzance/Cardiff will still need to run, but services would go no further than Birmingham. They can be transferred to GWR and run with a new order of 802s.
So what you're suggesting is having about the same number of trains to and from Birmingham New Street, albeit stopping at more stations, but terminating at New Street instead of working through, taking up platform space which is at a premium. If you're still running Cardiff to Birmingham, then a separate Birmingham to Nottingham, why not just stick the two together as is the case now? The same applies to your other proposals.
HS2 will not serve Doncaster so why not extend Birmingham-Sheffield on to Doncaster and then Hull? These are places which are large enough to merit a direct service to Birmingham. And you may as well have that as a continuation of the Bristol/Plymouth to Birmingham service to save platform space at New Street.
Any journey starting from Coventry/Leamington/Banbury to Reading and beyond is still going to be faster on the existing route. There's no point stopping XC at Oxford because you'd still need services to run from Oxford to Reading and Reading to Basingstoke. Why go to the hassle of splitting it up?
In truth, XC isn't a proper long distance operator. Most of its journeys are over relatively short distances, with quite a lot of overlap. It combines these regional journeys together into longer routes largely for operational efficiency. Yes, this allows long distance journeys to be undertaken, but this is a useful bonus, not XC's reason to exist. Once HS2 comes along, a good proportion of those long distance journeys will move to HS2, but all that means for XC is that it can concentrate more on being a regional service. So more stops as you suggest, and certainly curtailing the northern part of the network, but still working through New Street, and with some new destinations added.