One of the things that would have made a big difference to me back when I was on prepay is a functioning web portal to check how much credit you have - mobile as well as desktop. I was with BG who have a web portal which presumably works under Netscape on Windows 98, because it certainly doesn’t work on any other platform.
It would have been very useful to be able to check my remaining credit while I was out, so I could top up the key/card on my way home instead of having to make a separate trip - even more so for people who didn’t have a PP shop just round the corner. And it would have saved me getting dressed and going outside in the snow twice a day in midwinter too (after a few instances of everything shutting off unexpectedly when the shops were closed, bonus points if the gas went off while the boiler was running and it just sat there making an alarming grinding noise all night, I got serious anxiety about keeping track of what was on my meters.)
I don’t know if smart prepay meters are going to be a thing, but if so you should be able to top-up online as well. With immediate effect, so the boiler doesn’t just sit there making an alarming grinding noise all night.
(It’s been years since I was on prepay so I may have mixed up details, such as what features that pointless web portal was supposed to have)