Reading again what the DfT are discussing/asking, and the best way to describe it is to place them into routes that start/terminate in Birmingham. E.g.:
1tph Plymouth to Birmingham
1tph Bristol Temple Meads to Birmingham
1tph Cardiff Central to Birmingham
1tph Bournemouth (? (I find it highly unlikely they will get rid)) to Birmingham
1tph Reading / Southampton to Birmingham
2tph Birmingham to Manchester Piccadilly
1tph Birmingham to York (?) or Newcastle (?)
1tph Birmingham to Edinburgh via York
1tph Birmingham to Nottingham via Derby
1tph Birmingham to Cambridge
Of course, you also have the possibility of new services / new routes and destinations. You also have the complexity of Birmingham New Street: any trains coming from the University direction are best going either towards Water Orton or, if capacity exists (it should do) then they could run trains out via Aston to Wolverhampton mostly to avoid reversals but also to create capacity via Sandwell & Dudley.
You could also argue, perhaps to the disgust of the West Midlands authorities, in favour of the Nuneaton to Coventry shuttle staying as exactly that and routing both services from the South Coast via Coventry with one train stopping in at Kenilworth per hour so as to provide both through trains to Birmingham (surely the real desire of Kenilworth passengers) and to free up more freight capacity via Solihull.
There is also going to the possibility of a significant capacity uplift caused by the availability of the Class 222 fleet (or at least a large amount of it); meaning that you could expect new destinations would be easily added and still being able to gain more stock for longer trains across the network.
You've also got the conundrum of Tamworth and Burton and how to deal with capacity issues on these routes, assuming continued growth. Some form of fare supplement for Tamworth, Burton and Derby on any CrossCountry trains that stop there may help but even more so if you cause the West Midlands service to Nottingham to depart from Birmingham a couple of minutes behind an Edinburgh / Leeds / York / Newcastle that stops at those stations. You could mark them as "Cross Country Express" as opposed to just "Cross Country" and those services attract fare premiums between Birmingham, Tamworth and Derby; between Stoke, Macclesfield, Stockport and Manchester; between Sheffield, Wakefield and Leeds and any other chronic routes.
You would hope that Cambridge to Birmingham would gain 4/222 or 4/220 stock so as to become a proper Intercity service; and leaving Anglia's franchisee to deal with the Stansted - Cambridge route half-hourly where both would logically extend to Norwich should not be a problem and a change of trains in Cambridge is hardly a major task.
What's the best plan? As far as I can see, it would be to make the Cardiff service a Voyager/Meridian route as well and extend that through to somewhere like Leeds / York; whilst then getting the Bristol 'relief' hourly service to route to Nottingham. The Cardiff service would have less calls (e.g. Newport, Chepstow, Gloucester, Cheltenham, Birmingham New Street); whilst the Bristol - Nottingham would be 5 minutes behind it at Cheltenham and stop at Worcester Parkway, Bromsgrove (outer Birmingham connections), University (if needed) and New Street before then stopping at Tamworth, Burton, Derby, Long Eaton and Nottingham. The West Midlands franchisee would be given an hourly all-stations path for places like Wilnecote, Attenborough and Beeston.
The Birmingham - Cambridge would need only call at Coleshill, Nuneaton and Leicester before continuing off to Peterborough; whilst the Manchester services would be amended. I'd be inclined to suggest that one Liverpool service operated by West Midlands be diverted to Preston (creating a link for stations such as Hartford to Warrington and also including stations such as Leyland); whilst one Cross Country service operates to Liverpool. This would leave a Manchester gap but could be filled by an additional service.
Consider:
1tph 5/221 Cardiff Central to York (Newport, Chepstow, Gloucester, Cheltenham Spa, Birmingham New St, Derby, Chesterfield, Sheffield, Doncaster, York.)
1tph 4/222 Bristol Temple Meads to Nottingham (Bristol Parkway, Cheltenham Spa, Worcester Parkway, Bromsgrove, University, Birmingham New St, Tamworth, Burton upon Trent, Derby, Long Eaton, Nottingham.)
1tph 8/220 Plymouth to Manchester Piccadilly (Totnes, Newton Abbot, Exeter St Davids, Tiverton, Taunton, Bristol Temple, Bristol Parkway, Cheltenham Spa, Birmingham New St, route via Aston to Wolverhampton, Stafford, Stoke on Trent, Stockport, Manchester Piccadilly.)
1tph 8/220 Bournemouth to Edinburgh Waverley (Brockenhurst, Southampton Ctl, Southampton Airport, Winchester, Basingstoke, Reading, Oxford, Banbury, Leamington Spa, Coventry, Birmingham Intl, Birmingham New St, Tamworth, Derby, Sheffield, Wakefield Westgate, Leeds, York, Darlington, Durham, Newcastle, Berwick, Edinburgh Waverley.)
1tph 5/221 Southampton Central to Liverpool Lime Street (Southampton Airport, Winchester, Basingstoke, Reading, Didcot Parkway, Oxford, Banbury, Leamington Spa, Kenilworth, Coventry, Birmingham Intl, Birmingham New St, Smethwick Galton Bridge, Wolverhampton, Stafford, Crewe, Runcorn, Liverpool South Parkway, Liverpool Lime St.)
1tph 4/220 Cambridge to Manchester Piccadilly (Cambridge North, Ely, March, Peterborough, Stamford, Oakham, Melton Mowbray, Leicester, Nuneaton, Coleshill Parkway, Birmingham New St, Wolverhampton, Stafford, Stoke on Trent, Macclesfield, Stockport, Manchester Piccadilly.)
It takes now around 3h45 to do Ely to Manchester on East Midlands Trains; if you were smart with pathing a Cambridge to Manchester via New Street on a proper Intercity train could take as little as 4 hours; or just a couple of minutes over the existing Ely to Manchester journey time.
Consequential alterations:
1tph vice 2tph Birmingham New St to Liverpool Lime St operated by West Midlands
1tph new Birmingham New St to Preston operated by West Midlands
1tph new Birmingham New St to Leicester stopper operated by West Midlands
1tph new Birmingham New St to Nottingham stopper operated by West Midlands