We have had these discussions before. Liverpool, Manchester and Preston don"t each need a service every three hours to Plymouth just so some people don't have to change trains.
Liverpool had a train every half an hour to Birmingham until March 2020 that had connections for both Cross Country routes further south. Preston one every hour.
Maybe XC switching Manchester for Liverpool on the Bristol / Exeter service makes sense post HS2 in place of the LNR service but it will need to continue to serve Liverpool South Parkway, Runcorn and Hartford.
It is essentially only possible to run half hourly from Birmingham to Bristol - one train from the route through Derby and one train from the route through Stafford, preferably the same destination all day and convenient connections to the other places.
^^ Best comment on this thread ^^
XC is all about opportunity cost. There are a limited number of paths through Birmingham, so adding any new place to the service pattern is probably going to come at the cost of another place that loses its service.
So if you want to run an hourly Liverpool/Hull service then that means removing an hourly service from either Manchester or Newcastle. If you want to run an hourly Portsmouth/Brighton service then that means removing an hourly service from Bristol (or at least a bi-hourly service from Southampton).
Slice it up however you want but there aren't going to be a lot of additional paths through New Street to permit anything much more ambitious than what we currently have.
If you want to pay Paul then you're going to have to rob Peter. Maybe there's an argument that Paul is more deserving of the money than Peter, sure, but the starting position is effectively "half hourly from Birmingham to Stafford/ Sheffield/ Reading/ Bristol", and any additional destinations come at the cost of existing ones (e.g. if we keep two trains per hour then Manchester must lose out for Liverpool to gain).
And my person preference is to keep things simple with hourly and half hourly services - trying to run patterns of less frequent services to tick as many boxes as possible is a recipe for disaster (e.g. giving Plymouth a service to Liverpool, Carlisle, Manchester, Leeds, Doncaster and Nottingham might mean an hourly Plymouth - Birmingham service but how are you finding paths for these irregular services further north? Will there be a six-hourly path into all of these places? Which is unused in other hours? Put the crayons down.
Yet again, what about Bournemouth and Southampton? Why does their current well used hourly XC service suddenly get abandoned, yet Brighton gains one, using a route already known to have no capacity for XC?
I thought that the aim of the thread was to come up with as many contrary suggestions as possible (putting Liverpool, Blackpool, Bradford, Barnsley, Scarborough, Holyhead, Hull etc - and Brighton - always Brighton - on the Cross Country network) whilst ignoring the long established routes that people currently rely upon (and not just to go beyond Birmingham, but to do regional journeys like Basingstoke to Oxford - but such ordinary demands appear a lot less fun than getting the crayons out and randomly linking places far apart (see also "Cardiff - Birmingham - Aberystwyth"!)
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Wondered where that was going for a second or two...