What I would like to see is nationwide touch in and where you change and out, as now in London. Paper tickets phased out. Pricing based on "fare miles". No routing guide. Pay for the route you choose to take.
For example, Reading-Paddington could be more "fare miles" than real miles. Inverness-Thurso could be fewer fare miles than real miles. Sensible zonal pricing, eg Portsmouth & Southsea to Portsmouth Harbour would be zero fare miles, except for a journey within the Portsmouth zone.
Euston-Preston could not be more than Euston-Lancaster.
Price per fare mile degressive in tiers, eg up to 25 miles, 26th to 50th mile, etc.
Sensible stretches of each diagram priced at "standard", "off-peak", or "premium", colour-coded in timetables. For example, an early train from Hereford to Paddington could be off-peak for the Hereford to Worcester stretch, standard Worcester to Kingham, and premium Kingham to Paddington.
Four classes of fares: First class (only on proper big trains); Second class (what is now called first class on the likes of Turbos); Third class (what is now called "standard"); and Fourth class (pacers).
No seasons, but discounts for people who travel more than stated quotas of fare miles in a given (say) month.
Obviously, no return or "advance" fares. Separate charge for seat reservations. No railcards. Discounts for groups touching in and out together (and no, I don't know how that would be technically possible!)