There has been considerable discussion about them replacing 323s. Its not a mad plan when your consider the latters age and the ability to shuffle round stock to avoid reduction in capacity because they wouldn't be doubled up. Its not seem to got past early stages within DfT, the owner and Northern. Brum to Manchester is popular with enthusiasts (myself included) but its never been discussed seriously within the industry, at least not publically.
There is a sort of paralysis between DfT, Treasury, GBR and some Operators, where it appears nothing is likely to be ordered until 2026 or 2027. Many reckon it will take at least 3 years for any order to enter service (and nearer 5 years min if a new design that needs testing and certifying).
Clearly that means no new trains until at least 2029, maybe 2030 or even 2031.
Now if factor in passenger growth (and recent ORR quarterly figures show operators growing passenger numbers increasing by between 2% and 15% in a year). Or if you compare to 2019 then shows some operators are at 110-120%, whilst others are only around 85-90%. Now clearly if one projects average annual increase of 6% over next 5 years (until earliest new trains will arrive), then clearly unless most operators have vast amounts of slack they are going to need to bring back trains from storage.
Southern lost all its suburban 455s without replacement, South Eastern lost networkers (but did get 707s, although rather less than they lost). Northern has an old fleet too. So the question is who runs out of capacity first and manages to convince DfT that its fleet needs to grow.
From a PR point of view (and it might be date when media are focused) 4th December is when ORR have said they will publish both Regional Rail Usage and Annual station usage reports. It isn't going to look good if DfT ministers sit there are say we know about big growth locally, but decided passengers don't matter and not doing anything about more rolling stock, as they say watch this space.