as has been said, wiring the XC network has a very high CBR, and both routes between Leeds & Sheffield, as well as Sheffield-Doncaster, seem likely to get done in CP6. Plus Hull Trains' desire to wire from Temple Hirst Junction to Hull, even Hull could end up back on the XC network.
That said, the whole XC network could do with rationalising. There is no point whatsoever in running SW - Scotland services via the ECML, as it imposes a huge time penalty for doing so, and is already deeply uncompetitive with air services on that journey. Therefore, services from the South West should terminate at Newcastle. Additionally, I'm unconvinced of the benefits of extending XC services to Penzance compared to a more limited FGW Bristol-Penzance service. Also the Leicester-Cambridge line is always highly crowded, and could do with being made half-hourly.
Therefore, I'd hope for a timetable approximating:
1tph Manchester Piccadilly - Bournemouth
1tph Newcastle - Southampton (via Doncaster)
1tph Newcastle - Plymouth
1tph Lincoln - Swansea
1tph Nottingham - Cardiff
2tph Stansted Airport-Birmingham New Street
Additionally, some replacements of existing XC services
West Coast
1tp2h Bristol Temple Meads - Glasgow Central
1tp2h Bristol Temple Meads - Edinburgh Waverley
1tph Bristol Temple Meads - Manchester Piccadilly
East Coast
1tph Leeds - Aberdeen