I would start with the current WCML services that can be transferred directly to HS2:
1tph Liverpool via Stafford
1tph Manchester via Stoke-on-Trent
1tph Manchester via Crewe/Wilmslow
1tph Glasgow non-stop to Bank Quay
1tph North Wales via Crewe (although it would be good to have North Wales electrified by then, at least to Chester!)
These can be therefore taken off the WCML timetable, which leaves:
3tph WCML Birmingham, via Rugby (1), Watford (2), Milton Keynes (3)
As the Milton Keynes service currently continues to Scotland, I would duplicate that on both HS2 and WCML, and combine the Rugby and Watford services on the WCML. The WCML Scotland via Birmingham could have a stop added at Stafford, as it is no longer the fastest service and Stafford would provide more connectivity. I would put 3tph on HS2 London-Birmingham, as suggested above, all stopping at Interchange but 1 missing OOC.
I would leave the Manchester via Stoke and Milton Keynes on the WCML, and have 2tph going to Manchester on HS2 all the way, both stopping at Manchester Airport (pick-up/set-down only?) and 1 stopping Crewe and Birmingham Interchange.
I would take the Blackpool North Service off the WCML and put it on HS2, replacing the Rugby stop with Birmingham Interchange.
Extra services:
1tph extra Edinburgh/Glasgow via Crewe and Birmingham Interchange
1tph extra Liverpool stopping Birmingham Interchange but then fast to Runcorn
(extra 2tph Birmingham and 2tph Manchester already mentioned)