I'd expect route clearance on the whole WCML for HS2 classic compatible stock in case of engineering work.I think it's not unlikely residual WCML IC would use 200m HS2 units for commonality. Would also provide for flexibility in the event of engineering work or incidents on HS2.
Future stock also depends on how staff post HS2 is handled, the currently 5 1/2 hour long Euston to Edinburgh run via Birmingham (staying after HS2) means that Euston can't be the only staffing point unless TPE drivers are used for the 1tp2h to Edinburgh. Birmingham/Wolverhampton wouldn't be a bad pick but the Manchester service doesn't stop there.
Either way I'd imagine the Avanti Hitachis will be staying, any news on them?