In an ideal world I'd have:
North - South (Thameslink)
East West (Crossrail)
North West - South East (Bakerloo line extended to run Watford to the Dartford SE lines)
North East - South West (the "Great Northern" services using the Moorgate branch extended via the Waterloo & City line to cover SWT suburban services round Richmond/ Kingston/ Brentford)
Complete pie in the sky, and completely impractical, I know, but...
There was once a plan to extend the Bakerloo to Camberwell (back when it did run from Watford). I could see it going onwards to Peckham, New Cross and Lewisham after that.
I doubt you'd be able to enlarge the tunnels on the Drain to fit 313s through, but it might be possible to extend the Docklands Bank service through to Waterloo, perhaps with an extra station at Blackfriars (where the Drain goes under the river). However, going beyond Waterloo might be tricky without doing something about the lift shaft (BTW, an M7 fell down that once).
The one I'd really want to see would be the HS1 domestic services reversing at St Pancras and running through onto the WCML, taking over the Northampton fasts perhaps. Then, when and if the HS2 Heathrow spur gets built, they could take over that, and perhaps an Aylesbury-Brackley-etc HS2 domestic service. The other would be a new Thameslink tunnel, as proposed in an old issue of
Rail I saw once, that would run from St Pancras to Blackfriars via Grays Inn Road and Chancery Lane, thus opening up the Moorgate route for an extension of the Euston d.c. lines and City Thameslink as a major terminus for South East suburbans. Of course, this would negate all the work done already, but might be easier in the long-run.
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But the vast majority of tickets sold at Stansted are for Tottenham Hale or London Terminals. While there are indeed folk travelling across London, these are actually few and far between. Nor are the many passengers necessarily travelling to or from Bank and "the City".
That could be because passengers from overseas (the majority who buy at Stansted) don't know that you can buy through tickets to anywhere else. I couldn't tell you without a survey, of course.
The vast majority of business passengers will be going for meetings/conferences in London, which usually means the City or Docklands. Those going to hotels using HEx (including tourists) tend to take taxi or tube connections from Paddington. There, you would have to buy a separate ticket anyway. Once again, you would need to do a survey.
It's
outgoing passengers I'm considering, the ones who buy tickets at their local stations. That's one of the quirks of airport travel, people flying out tend to buy through tickets, which means that airport owners going on their own figures may not consider this - the same mistake the Beeching survey made about branch lines. Without access to the database, I couldn't possibly know what the figures are, but there must be a vast discrepancy. Does anyone know any more?