Some interesting comments and suggestions - but there are problems & consequences if some of the suggestions were implemented.
So - just my opinions.
1. Lime Street site (at surface level) is almost impossible to enlarge to take full length HS2 trains. It will only be able to accept shorter length "classic compatible trains". An 11 coach Pendolino is about the longest train it will ever be able to accept.
2. Central High Level was too short to accept even something like a 9 coach Pendolino.
3. Exchange site is still largely useable (if you spend enough money), but any new line would almost immediately need a long tunnel to exit from the city central region. The frequency of local services on the Southport, Ormskirk or Kirkby lines is such that none of them will ever be suitable for fast, long-distance trains. 4-tracking as far as Ormskirk or Kirkby would also be very difficult & expensive.
So - if full length HS2 trains are ever to come to Liverpool, a completely new station - probably underground - is almost the only solution.
Station capacity. Yes, given enough money, Wapping Tunnel might be reinstated as part of a link from Edge Hill to Central Low Level. But, diverting locals away from Lime Street would lose connectivity with the long distance services to London, Birmingham, etc. - and I suggest that would be very unpopular.
And what would be the benefits for existing passengers - they would get less attractive trains with, probably, fewer seats than a Class 319 or 323 - and then be faced with the already overcrowded Central station. On a simple route network, a single island platform could cope with more trains per hour. But the "City Lines" network is anything but simple. Unlike much of the current Merseyrail Electrics lines, the City lines have complex interactions with other routes. Trying to push many of these extra City Lines trains into Central would wreck punctuality / timekeeping. Widening the tunnel at Central would be hideously expensive, and the entrance / booking hall area would also need to be enlarged. Continue to Moorfields, and you have long walks and 2 long escalators before you reach the exit - in a part of the city that does not seem very welcoming at night.
And what is the demand for linking services across Liverpool city centre ? The most popular destinations would probably be Chester & Birkenhead - but you would still have to change, at Central rather than Lime Street. Next - Southport, but that already crosses the city to/from South Parkway/Hunts Cross. And I doubt that there is much demand for City Lines passengers to travel to/from the Ormskirk /Kirkby lines (except once a year on Grand National Day)
So - a complex problem - with no easily affordable solutions - and a danger of making things worse, not better....
