In an ideal world, a lot of the wastage in the railway (the stuff Old Timer does such a good job of highlighting, with actual experience and knowledge) would be reduced.
We would get better value for money for projects, like station rebuilds and refurbishments (so it won't cost millions to put in lifts or extend a platform) so more improvements could be made.
We wouldn't seek to renationalise, but if a Government was serious about encouraging people out of their cars, then the railway might get more of a subsidy - because fares are too high here (IMO) and just because some operators do ridiculously cheap deals with advance fares, like the airlines, it isn't the solution when public transport is more of a necessity than flying (generally).
With the need to invest more to copy with peak travel, that will mean more scope to running and encouraging more off-peak travel. Instead of just going with, often confusing, advance tickets and deals (with different restrictions on different TOCs) we should be investing in smart ticketing solutions. Thus, you can simplify everything - and those who travel the most will automatically get savings, or loyalty discounts, over time. The 'more you travel, the more you save' logic would probably get more people on the trains, just as such schemes work for any other industry.
There's so much that can, and should be done, but I fear that what is going to happen is that fares just go up by a percentage every month and not a lot else. Obviously we'll get ongoing maintenance and after a certain time, when there's no longer any choice, some new rolling stock, but (besides HS2) there doesn't seem to be much thought about the future needs.
I'm still a great supporter of working from home, but don't believe that enough people will ever do that to make a big impact.