I think this is a much bigger issue, which has been largely missing from the talk about minor tweaks to fares. The odd system we have now - a large peak/off-peak differential (and very large on some routes), which is softened for those travelling to work by substantial season ticket discounts - was driven by the difficulty in coping with the peak demand. Take that factor away, and doesn't the whole reason for pricing tickets like this go away too?
Yes - as part of my FlexiSeason Study
((here))Melksham to Bristol - 1% (£12.70 v £12.60)
Melksham to Newport - 15% (£24.10 v £20.90)
Melksham to Salisbury - 29% (£19.00 v £14.70)
Melksham to Swindon - 35% (£10.80 v £8.00)
Melksham to Oxford - 216% (£56.80 v £18.00)
... and that Oxford figure is not a unique aberration - similar proportions for Melksham to Didcot, Reading and London. On Oxford and Didcot, there is no restriction on evening return on the cheaper fare - on Reading and London, there is.
On the routes where the peak premium is under, say, 25% there should be little issue with an anytime fare to simplify the system, positioned towards towards the lower end of the price bracket - say £22.00 to Newport, perhaps £16.00 to Salisbury. At the top of the range, you have a very real issue - even in the Melksham to Oxford fare went to £22.00 anytime, that's still a massive loss of income to the rail industry.
Perhaps pricing is not going to be so much on what the loadings are, but on what people are prepared to pay?
