I think that the Germans - or the French for that matter - would build a Y-shaped network starting from St Pancras, with new, longer platforms on the West side in place of the current EMR platforms. EMR trains would be diverted into/out of Euston via a new chord in the West Hampstead area and HS2 services would follow the existing Midland Main Line as far as Hendon, using the current Hendon lines from West Hampstead to Hendon, where the High Speed line would diverge to follow the West side of the M1 motorway, and run alongside - or close to it - all the way to Leeds, with Parkway-style stations at - or just outside - Northampton, Leicester, Derby/Nottingham and Sheffield (Meadowhall). The Western branch would diverge near Lilbourne and follow the M6 all the way to Carlisle, with Parkway stations at Coventry/Nuneaton, Birmingham (somewhere in the Gravelly Hill/Aston area....ideally incorporating an interchange with the Cross-City line),Wolverhampton/Walsall, Stoke-on-Trent, Warrington/Wigan, Preston, Lancaster and possibly Carlisle , should the HS line continue into Scotland. There would be chords to the South of Crewe for Liverpool and Manchester services, South of Preston for Blackpool services and South of Lancaster for Furness and Lake District services.
Ideally the Parkway stations would be situated where the line crosses the Northampton Loop just South of Hunsbury Hill Tunnel; the Coventry-Nuneaton Line at Longford; the Derby-Nottingham line at Nuneaton; The Cross-City line at, or near Aston, the Wolverhampton-Walsall line somewhere between Darlaston Jn and Portobello Jn. (connecting with a reinstated Wolves-Walsall service); the 'Potteries Parkway' near Madeley, with a connecting service over a reopened line from Stoke; the Liverpool-Manchester line at Newton-le-Willows; the Preston-Blackburn line at Bamber Bridge and the Lancaster Parkway station would be at Galgate/Bay Horse. On the Eastern branch, stations would be located where the HS line crosses the Birmingham-Leicester line at Narborough; the Derby-Nottingham line at Long Eaton; and the obvious interchange at Meadowhall.
Whether or not the Eastern branch would continue Northwards from Leeds would depend on the finance available, given that there is already a fairly fast route between York and Darlington.....and whether or not either or both branches would continue into Scotland would depend on politics and economics; particularly on whether - and how much - the Scottish government was willing and able to contribute financially.