I have always thought it would be far better if all the mainlines to London joined in one big hub. People say it would be too complicated and take too long to navigate, but it would be less complicated and be far quicker than the current situation of travelling between various stations on the underground. It is just that we are used to that.
Rather than combining it into one station though, I would have about 10 stations along the same street neighbouring each other, perhaps even under the same roof. These could be called “London West”, “London Northwest“, ”London North”, etc, depending on where the trains went. Each would have its own entrance and exit to keep passenger flows simpler. Essentially it would be one station, but considered separate operationally and from a signage/passenger flow point of view. Good signage and a fast way to travel into, out of, and around the station would obviously be key, but you’d expect it to be state of the art, being the main London station.
It would make travelling within and across London so much easier for both long distance and commuting travel. All of London’s transport would be designed around it, so almost all journeys would need only one change, and no underground etc.
If it were designed from scratch (not that London was ever really “designed”) then I suppose it would be in a central location, with the lines heading directly towards it. If it had to be built now, the easiest way would probably be for HS2 to pick up the GWML and WCML at Old Oak Common, the MML to pick up the Chiltern around West Hampstead, and HS1 to pick up the GEML around Stratford, and take them all to Kings Cross St Pancras area. Then the SWML to pick up the BML at Clapham Junction, and run them on to join Thameslink somewhere around Waterloo. Dig a big tunnel and follow the Thameslink route to join it all up. The station could either be around the current St Pancras area, or by the Thames.
But by this point, it might be easier to build a station somewhere in the countryside, and move London around it instead.