I think the prepaid card market is ripe.
I mean, I know monzo started as prepaid & then moved mainstream, but I think there are many folks (I know a few) who prefer to use a prepaid card for separating their cash for specific stuff, giving to their kids etc, and aren’t inclined to take out a separate bank account, either because of mistrust of banks, or concerns about what “the social” will say, or even due to a lack of the kind of documentation Monzo tend to need (i.e. driving licence, passport). Weirdly, the fees are never a concern (even though some prepaid cards charge you for everything under the sun).
If Monzo could create a prepaid version, similar app to what we have now (minus direct debits and whatnot), keep the KYC checks to a minimum (or expand the documents to things these folks might actually have), and have a simple low fee (just to make it worth doing), then I’d imagine that would work.