Making multiple overnight breaks of journey on a single ticket is so niche that, while I'm sure it's of value to the tiny number of people who do it, I don't think it is necessary to even consider it in any new system if not considering it provides any benefit e.g. of simplification. Same day breaks of journey are a very common use case (e.g. to pop to the shops between trains, which may or may not involve getting the next possible train) and I believe those should be permitted on all tickets, Advances included. One overnight BoJ is common enough to allow it. Beyond that it is extremely niche.
For what it's worth, though, I am not opposed to retaining the ability to purchase a return ticket, but it would simply be priced at twice the relevant single fare. The benefit of doing so would continue to be the month long validity. That is not incompatible with single leg pricing - the core of which is simply the principle that there is no discount for making two journeys on the same ticket.
Personally, I'd make long-distance singles valid for either two or five days, which would give you better terms on the outward than you presently have other than an Anytime. You can't viably break a PAYG journey, but then in that case a paper ticket may make more sense, just as there are use-cases in London where PAYG is better avoided in favour of a paper Travelcard.
