This discussion has been had many many times before...
If NT and TPE where to merge later on down the line to have "Northern CityLink" or keep a seperate brand such as TPE etc.
The routes we (I) came up with last time are;
South TPE to operate through over the CLC lines. Liverpool L St to Cleethorpes (12 units initially with no strengthening, 20 with core units added between Doncaster and Livepool, potentially portion working to include Selby or Hull via Goole from Doncaster)
If working to/from Hull via Goole and Cleethorpes (Providing a significanly higher quality service to Hull for links to London at Doncaster and at Sheffeild) this would primeraly provide a convenient place for the 2nd unit to go, other options would be Selby, that isn't too bad, or Adwick (adds extra mainline crossings) if one unit can cross the mainline as current timetable, one could split the service North/East of Doncaster at Hatfeild & Stainforth.
Buxton to provide 2tph peak and 1tph off peak, fast to Hazel Grove calling Stockport Only, again, potential portion working with South TPE, or used to provide 2tph TPE between SPT and MAN then onwards to Lime Street but more dowbtful as this would gain the best potential pathing terminating at Piccadilly (also better peak response as this would be the most peakey route), 5 - 10 Units
Calder Valley and/or Leeds - Settle - Carlisle (Remaining units, interworking with nicely refurbished 158 units)